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.