Launch the game with the -allowdebug switch, and run mission_reload in the console. That should show if anything is wrong with the mission file.

so its say Mission file "missions/backyard.txt" has invalid settings for game mode coop
MissionManager: Mission file "missions/backyard.txt" does not have valid data for any supported game mode
so how can i fix it?

Check if there are any misspelled filenames or missing quotation marks or brackets. If you can't find anything, post your mission file.

i cant so i post my mission file.

// Mission files describe the metadata needed by campaign-specific add-ons so they can be
// integrated into Left4Dead. The data in this file is used by the game UI, matchmaking and server.
// Although you may provide multiple Campaigns in one add-on by putting more than one .TXT file
// in the missions folder, it's generally a good idea to stick to one per add-on.
//
// HOW TO DEBUG MISSION FILES:
// In the console set "developer 2"
// Then type "mission_reload"
// This will make the game reload all the mission files and print out every chapter for every mode it
// finds. It's very useful to ensure that your mission file is being correctly read.

"mission"
{
// Use a short name, since it is used as a tag on the servers for matching
// your campaign when looking for a dedicated server. Generally it should
// be something unique. One suggestion to is use your initials and a short
// abbreviated name for your campaign. Avoid spaces and special characters.
// Do not change the name when you create a revision, as the matchmaking
// system will consider it a different campaign. Instead, use "Version" and
// "DisplayTitle" below to indicate revisions.
"Name" "Backyard"

// The version number is used when finding campaigns on dedicated servers
// and in matchmaking. If you try to connect to someone in matchmaking
// who has a newer version, you will be directed to download the new
// version. You must increment this to the next integer (whole numbers)every
// time you release an update. (I.E. 1, 2, 3, 4, etc.)
"Version" "1"

// Author is displayed in the UI when people see others playing your
// campaign.
"Author" "(DA)dbsoldier"

// Website is extremely important as people will automatically be sent
// to this URL if they try to join a friend running your campaign. This
// should be the home page for your campaign and it should provide a
// description, a link to download it, and installation instructions.
"Website" "http://www.christi.dk"

// This name is used when referring to the campaign in the UI.
"DisplayTitle" "BackYard (1.0)"
"Description" "The Survivors Have to get to the last train Before Its to late"

// Vmt shown behind the end credits when your campaign completes.
// Note: This item needs to have "vgui\" at the front. It is assumed
// for the poster and the map thumbnails.
"OuttroImage" "vgui\OutroTitle_backyard"

// Loading poster data
//
// Note that "posterTitle" and "posterTagline" are often blank as
// the poster usually includes these in the poster image itself.
// If you do not provide a poster, a generic default will be used.
"poster"
{
"posterImage" "LoadingScreen_backyard"
//Note L4D2 does not position player names over the poster.

}

// The modes section lists each of your campaign maps and each mode
// they support. Depending on how you set up your campaign maps,
// you can reuse the same bsp for different modes as is the case with
// coop and versus in this example. This requires setting up entities
// that show up or operate differently for different modes.
//
// The following modes are supported: "coop", "versus", "survival"
//
// Number each map section starting with "1"
//
// "Map" is the name of the bsp of the starting map. (do not include
// an extension)
// "DisplayName" is the name to use for the chapter of the map.
// "Image" is the image used for the chapter in the lobby and
// settings.

"modes"
{
"coop"
{
"0"
{
"Map" "l4d_gp_1"
"DisplayName" "On The Way"
"Image" "maps/l4d_gp_1"
}

"0"
{
"Map" "l4d_gp_2"
"DisplayName" "Follow The Tracks "
"Image" "maps/l4d_gp_2"
}

"0"
{
"Map" "l4d_gp_3"
"DisplayName" "Trainstation"
"Image" "maps/l4d_gp_3

}
}

'"versus"
{
0"
{
"Map" "l4d_gp_1"
"DisplayName" ""
"Image" "maps/l4d_gp_1

"0"
{
"Map" "l4d_gp_2"
"DisplayName" ""
"Image" "maps/l4d_gp_2"
}
}

"survival"
{
"1"
{
"Map" "l4d2_down_round"
"DisplayName" "BackYard Survival"
"Image" "maps/l4d2_down_round"
}
}
"scavenge"
{
"0"
{
"Map" "l4d_gp_1"
"DisplayName" ""
"Image" "maps/l4d_gp_1"
}
}
}

}

The game doesn't like those "0" entries for game modes you don't use. Either remove them, or comment them out with //.

i have tryed both things now it still not work but i post my new edit mission file.

// Mission files describe the metadata needed by campaign-specific add-ons so they can be
// integrated into Left4Dead. The data in this file is used by the game UI, matchmaking and server.
// Although you may provide multiple Campaigns in one add-on by putting more than one .TXT file
// in the missions folder, it's generally a good idea to stick to one per add-on.
//
// HOW TO DEBUG MISSION FILES:
// In the console set "developer 2"
// Then type "mission_reload"
// This will make the game reload all the mission files and print out every chapter for every mode it
// finds. It's very useful to ensure that your mission file is being correctly read.

"mission"
{
// Use a short name, since it is used as a tag on the servers for matching
// your campaign when looking for a dedicated server. Generally it should
// be something unique. One suggestion to is use your initials and a short
// abbreviated name for your campaign. Avoid spaces and special characters.
// Do not change the name when you create a revision, as the matchmaking
// system will consider it a different campaign. Instead, use "Version" and
// "DisplayTitle" below to indicate revisions.
"Name" "Backyard"

// The version number is used when finding campaigns on dedicated servers
// and in matchmaking. If you try to connect to someone in matchmaking
// who has a newer version, you will be directed to download the new
// version. You must increment this to the next integer (whole numbers)every
// time you release an update. (I.E. 1, 2, 3, 4, etc.)
"Version" "1"

// Author is displayed in the UI when people see others playing your
// campaign.
"Author" "(DA)dbsoldier"

// Website is extremely important as people will automatically be sent
// to this URL if they try to join a friend running your campaign. This
// should be the home page for your campaign and it should provide a
// description, a link to download it, and installation instructions.
"Website" "http://www.christi.dk"

// This name is used when referring to the campaign in the UI.
"DisplayTitle" "BackYard (1.0)"
"Description" "The Survivors Have to get to the last train Before Its to late"

// Vmt shown behind the end credits when your campaign completes.
// Note: This item needs to have "vgui\" at the front. It is assumed
// for the poster and the map thumbnails.
"OuttroImage" "vgui\OutroTitle_backyard"

// Loading poster data
//
// Note that "posterTitle" and "posterTagline" are often blank as
// the poster usually includes these in the poster image itself.
// If you do not provide a poster, a generic default will be used.
"poster"
{
"posterImage" "LoadingScreen_backyard"
//Note L4D2 does not position player names over the poster.

}

// The modes section lists each of your campaign maps and each mode
// they support. Depending on how you set up your campaign maps,
// you can reuse the same bsp for different modes as is the case with
// coop and versus in this example. This requires setting up entities
// that show up or operate differently for different modes.
//
// The following modes are supported: "coop", "versus", "survival"
//
// Number each map section starting with "1"
//
// "Map" is the name of the bsp of the starting map. (do not include
// an extension)
// "DisplayName" is the name to use for the chapter of the map.
// "Image" is the image used for the chapter in the lobby and
// settings.

"modes"
{
"coop"
{
""
{
"Map" ""
"DisplayName" ""
"Image" "maps/"
}

""
{
"Map" ""
"DisplayName" " "
"Image" "maps/"
}

""
{
"Map" ""
"DisplayName" ""
"Image" "maps/

}
}

'"versus"
{
"
{
"Map" ""
"DisplayName" ""
"Image" "maps/

""
{
"Map" ""
"DisplayName" ""
"Image" "maps/"
}
}

"survival"
{
"1"
{
"Map" "l4d2_down_round"
"DisplayName" "BackYard Survival"
"Image" "maps/l4d2_down_round"
}
}
"scavenge"
{
""
{
"Map" ""
"DisplayName" ""
"Image" "maps/"
}
}
}

}

Remove the whole content for the entries, even the coop, versus and the scavenge ones.

more like this.
// Mission files describe the metadata needed by campaign-specific add-ons so they can be
// integrated into Left4Dead. The data in this file is used by the game UI, matchmaking and server.
// Although you may provide multiple Campaigns in one add-on by putting more than one .TXT file
// in the missions folder, it's generally a good idea to stick to one per add-on.
//
// HOW TO DEBUG MISSION FILES:
// In the console set "developer 2"
// Then type "mission_reload"
// This will make the game reload all the mission files and print out every chapter for every mode it
// finds. It's very useful to ensure that your mission file is being correctly read.

"mission"
{
// Use a short name, since it is used as a tag on the servers for matching
// your campaign when looking for a dedicated server. Generally it should
// be something unique. One suggestion to is use your initials and a short
// abbreviated name for your campaign. Avoid spaces and special characters.
// Do not change the name when you create a revision, as the matchmaking
// system will consider it a different campaign. Instead, use "Version" and
// "DisplayTitle" below to indicate revisions.
"Name" "Backyard"

// The version number is used when finding campaigns on dedicated servers
// and in matchmaking. If you try to connect to someone in matchmaking
// who has a newer version, you will be directed to download the new
// version. You must increment this to the next integer (whole numbers)every
// time you release an update. (I.E. 1, 2, 3, 4, etc.)
"Version" "1"

// Author is displayed in the UI when people see others playing your
// campaign.
"Author" "(DA)dbsoldier"

// Website is extremely important as people will automatically be sent
// to this URL if they try to join a friend running your campaign. This
// should be the home page for your campaign and it should provide a
// description, a link to download it, and installation instructions.
"Website" "http://www.christi.dk"

// This name is used when referring to the campaign in the UI.
"DisplayTitle" "BackYard (1.0)"
"Description" "The Survivors Have to get to the last train Before Its to late"

// Vmt shown behind the end credits when your campaign completes.
// Note: This item needs to have "vgui\" at the front. It is assumed
// for the poster and the map thumbnails.
"OuttroImage" "vgui\OutroTitle_backyard"

// Loading poster data
//
// Note that "posterTitle" and "posterTagline" are often blank as
// the poster usually includes these in the poster image itself.
// If you do not provide a poster, a generic default will be used.
"poster"
{
"posterImage" "LoadingScreen_backyard"
//Note L4D2 does not position player names over the poster.

}

// The modes section lists each of your campaign maps and each mode
// they support. Depending on how you set up your campaign maps,
// you can reuse the same bsp for different modes as is the case with
// coop and versus in this example. This requires setting up entities
// that show up or operate differently for different modes.
//
// The following modes are supported: "coop", "versus", "survival"
//
// Number each map section starting with "1"
//
// "Map" is the name of the bsp of the starting map. (do not include
// an extension)
// "DisplayName" is the name to use for the chapter of the map.
// "Image" is the image used for the chapter in the lobby and
// settings.

"modes"
{

"survival"
{
"1"
{
"Map" "l4d2_down_round"
"DisplayName" "BackYard Survival"
"Image" "maps/l4d2_down_round"

now it work but it still say there are some errors.
KeyValues Error: RecursiveLoadFromBuffer: got EOF instead of keyname in file missions/backyard.txt
mission, modes, survival, 1,
KeyValues Error: RecursiveLoadFromBuffer: got EOF instead of keyname in file missions/backyard.txt
mission, modes, survival, (*1*),
KeyValues Error: RecursiveLoadFromBuffer: got EOF instead of keyname in file missions/backyard.txt
mission, modes, (*survival*), (*1*),
KeyValues Error: RecursiveLoadFromBuffer: got EOF instead of keyname in file missions/backyard.txt
mission, (*modes*), (*survival*), (*1*),

Gotta close thse brackets. You need four of them in the end. Here: }}}}

something like this.

"modes"
{

"survival"
{
"1"
{
"Map" "l4d2_down_round"
"DisplayName" "BackYard Survival"
"Image" "maps/l4d2_down_round"

}
}
}

}

now its work and thank you for the help and one more thing will it work for modes like versus and scavenge as well?

You'll have to add some entries back then. You can copy the survival one and change the mode name.

ok and again thank you for the help