A mapper on the Workshop has find the fix for the UNKNOWN MAP ERROR, I experimented this too with some maps in Workshop.

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The fix by ┊JD Army┊G7SKIM:

UPD TIP - IF "Map doesn't exist" SOLUTION

1. Subscribe the map!
2. For some reason Steam downloads files in the wrong folder.
In order to make it work, you need to manually copy the files from one folder to another.
3. Go to the directory:
C:\Program Files (x86)\Steam\userdata\...\ugc\referenced\SET_OF_NUMBERS FOLDER
or
C:\Program Files \Steam\userdata\...\ugc\referenced\SET_OF_NUMBERS FOLDER
4. EDIT FROM ORIGINAL SOURCE: In one of these folders you will find files with this extention .cmwsdk
Copy them all.
5. Copy this file to a directory:
C:\Program Files(x86)\Steam\SteamApps\common\chivalrymedievalwarfare\UDKGame\CookedSDK\__CMWSDKFiles
or
C:\Program Files\Steam\SteamApps\common\chivalrymedievalwarfare\UDKGame\CookedSDK\__CMWSDKFiles

If there is no folder "__CMWSDKFiles" create it manually.
6. Now you can start the game and open the map through the console!
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Source: https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=198310435

If you want to thank this great mapper personally, here is its profile: https://steamhost.cn/steamcommunity_com/id/g7skim

Best Regards
JamyzGenius/JFG/JamyzPunchThis