This error appears when trying to Compile your map and will entirely fail and stop the compiler. To fix this, all you must do is find the missing prefab and remove it from your map.
In my case, this was my error:
ERROR: 'D:/SteamLibrary/steamapps/common/Call of Duty Black Ops III/pc/..\map_source\_prefabs/zm_tuvong/bunker/bunker_2.map' is missing 'iwmap' version specification
I had to find where "bunker_2.map" is located and remove it. This turned out to be an empty prefab that didn't exist and was named improperly.
That should fix the error!