Thanks for your interest!
When I download my app with download feature(not distribute, download iOS button), and open it (profile is already set and linked to this app) then this error message returns.
Thunkable needs to update for iOS 15 and iPadOS 15. In all likelihood, this might have already happened, but we probably won’t see it until 15 launches in September.
In the meantime, Thunkable apps probably won’t run on 15. I have the same problem on my iPad running iPadOS 15.