Hey there @King_Crazy! This seems to be an error with how Unreal is handling a UI component (or what is passed as a window). MacOS seems to be upset with it originating from a thread other than the primary. That said, I’ve never seen this outside of app development. Since Unreal generally handles all internal API calls, I didn’t even expect this to be easily done without a modified engine build.
This is a build for iOS, right?
Do you have a previous source control branch in which it did build? If so what has changed since then?
Try to package the game with just a blank map and leave your main map out, does the build fail in the same location?
At the risk of breaking things further, my first suggestion is to back your project up, and delete every folder with cached data. Saved, DerivedDataCache, and Intermediate and when you build, make a clean build, does the error remain the same or change?