Do you have any virus protection software that may be hindering it? Try to put your virus protection in Gaming Mode or disable it for the time being.
There’s also a chance that your map is corrupted.
Try this: Go to File > Export > save a .t3d file > Create a new map > File > Import > and select the .t3d file you just created.
Lastly, if none of those work: Delete sections/assets of the map and place them back on the map, so you are going to essentially rebuild the level, piece by piece. Keep track of what assets finally enable you to rebuild the map after deleting/replacing them just in case you have the same issue in the future.
Ok so if I delete everything… It works. If I go through and delete everything part by part, it doesn’t work. (So, everything beginning with A gets deleted. Build… Error. Everything with B… .etc.)