If you try to build iOS or tvOS projects in current versions of GMS2 after updating to last week's brand-new Xcode 12, then you will find this no longer works reliably. This is because the launch version of Xcode introduced some changes to the names of packages and other settings, which the already-released GMS2 2.3.0 was therefore not aware of.
The core issue is that UIKit.framework is not added to the project - although you may recognise this by seeing errors/warnings such as "Undefined symbols for architecture arm64:" and "MobileCoreServices has been renamed. Use CoreServices instead" inside Xcode itself. Whilst you can easily add this framework into your Xcode project and fix the issue if you just want to quickly test something, because your GMS2 runtime doesn't request the package, you would need to keep adding it into Xcode each build.
We have already addressed Xcode 12 support for the next GMS2 release (2.3.1), and so we wanted to share the files which will fix this in your 2.3.0 install also, avoiding the need to keep adding the framework manually yourself.
1. Close GMS2.
2. Download the .zip file attached to the bottom of this FAQ
3. Extract the zip and overwrite your existing files. However, this bit differs if you're on Windows or Mac...
Windows IDE users, move the .zip file into the root of your 188.8.131.521 runtime folder (e.g.: C:\ProgramData\GameMakerStudio2\Cache\runtimes\runtime-184.108.40.2061\ ) then right-click the .zip file and choose "Extract here", saying "Yes to all" when Windows asks if you want to overwrite the existing files. (Once you're done with the .zip you can delete it if you wish.)
Mac IDE users, double-click the .zip file wherever you downloaded it and Archive Utility will extract it to a new folder. Now open a second Finder window and navigate to /Users/Shared/GameMakerStudio2/Cache/runtimes/runtime-220.127.116.111/ . Back in the first Finder window, select the individual ios/tvos/yyc folders in turn, working your way through their sub-folders, and move all of the files from this window into the matching runtime sub-folder in the second Finder window. (Once you're done with the .zip and the empty new folder you can delete them if you wish.)
4. Start GMS2 again and load your project, then clean your project cache and finally build again. You should find all is now fixed.
Again, this fix is only for 2.3.0 - it will not work with 2.2.5 or older and we do not plan to offer the same fix for those old versions. Please either update to 2.3.0 and follow the above steps again, or simply downgrade to Xcode 11.