Ok, right now I start to become frustrated.
I try to upload many maps to Freebuild but this error keeps popping for different maps also.

Hi,
I got this map so when you dowloaded you must to "unzipp" the .sm2 files ifrom .rar files in your C: Documents>stronghold 2 >Map
After you may play this map
but I was trying too to put a map on the workshop
and it does not work , i don't have the same error like you , for me it is because :
when i opened stronghold 2 , of course , I was connected to Steam but when I clic on the "view term of service" or "steam workshop" I got a screen where I must installing STEAM ....
so i think it is because I put all my game on the E local disk...

an another strange thing When i found a script error on a map , i generally may correct it in the editor map ....but I must before copy paste the .sm2 map from the steamapp folder on my folder map on C: because i can't open any file with the editor from steam ..