I was getting this on an enterprise app built with Xcode 12.4 on Catalina. I rebuilt with Xcode 12.4 on an M1 Mac with Big Sur and the app now opens fine on iOS15. I used automatic signing on both - so it wasn't an issue with manual signing for me. I was and still am targeting iOS13 btw - the Xcode 12.4 on Big Sur doesn't have the iOS15 options that come with Xcode 13.

Okay, well I've discovered the client has created the developer account as an individual account not as an organisation - that explains why I can see App Store Connect but not the team in developer.apple.com Why the main owner/admin account won't work in Xcode (even on a brand new M1 Mac) is still a mystery but I am gonna get my client to migrate their individual account to an organisation account and add me as an app manager or developer and then it should all work.