Are you in a dedicated server environment, or a listen server environment?

I am just playing a locaol server with a friend, and for some reason the zombies are not spawning.

Same for me. No zombies are spawning, and I only have Redead enabled as an addon.

Ah alright! I finally figured it out! Some maps might not have the Redead spawn locations, and/or a generated nav mesh. You can actually modify any map to have redead spawns on it even if the map is not meant for redead! all you have to do, is:
1: start a local redead game on the map you want to edit.
2: Select class blah blah blah/
3: Open up your console. the ~ key.
4: Type in: sv_redead_dev_mode. You will be given 500 bones, and a couple of weapons/sweps.
5: Scroll until you find a swep called item placer or something like that.
6: Right click to change the item you want to spawn, such as an info_npcspawn.
7: Left click to spawn the item at the location you are looking at.
8: Once you are finished with putting the items in their locations, then open the console again.
9: Type in: sv_redead_save_map_config. Then restart the map.

If for some reason the zombies are spawning but are not moving, then follow these instructions:

1: Open the console.
2: Type in: nav_generate. (Of course, you must press ENTER after each message you type into the console :P) Your game will lag, but that is because it is generating a nav mesh for the NPC's and Bots to know where to go. When it is done, your map will automatically restart, and then you can play the game!

Sometimes after you type in nav_generate, it will say something like, "there are no walkable positions" or something similar to that. I have no idea how to fix this, but I just assume that the map will not work for nav meshes.

When in case of "No valid walkable seed positions. Cannot generate Navigation Mesh." look at the ground then in console :

sv_cheats 1
idle_autokick_enabled 0
nav_edit 1
nav_mark_walkable
nav_generate

Should be fixed.
Don't forget to nav_edit 0 after that ;)

or maybe its because you dont have the npc addon just a idea

FIX lT