I think I found a work around which is to launch the simulator first then launch Xcode then build the app. If you go the usual route your app will freeze. Also the preferences app will keep crashing at least on my system so can't sign in to test CloudKit syncing.

Had this issue a while back. I had to delete all the screenshots then log out. Log back in upload the screen shots and then it seemed to work and was able to submit my app.

I figured it out. I had to build one of my old projects with iOS 14. Once the simulator came up then I rebuilt my new project in iOS 15 and it came up with no errors.

You will need to migrate your existing core data store with NSPersistentHistoryTrackingKey enabled. This will then allow your old data to be sync'd using NSPersistentCloudKitContainer.