Ok, I had this problem too, it is easier to solve than it seems. Well, my project had a problem at first because I created it inside Google Drive, because it was a very long path, I should have been aware that I would find errors in the future, and in this case, it gave the fatal error. What I did was the following, quite simple in fact, I created another project from 0 on the desktop, I went to the original project and renamed the name of the level, then I migrated it. So in the game, in the maps and modes I assign my new level. I did the packaging and it worked just fine, I just had a slight rework to link the meshes in the variant plugin, but it was pretty quick.