Your Apple ID can only be used on devices running iOS 15.0 or later.
I have an older iPad Air and I get this message when I try to sign in with my wife’s Apple ID. But using my Apple ID I can sign in. It’s running 12.5.7, and that’s the latest update available.
Your Apple ID can only be used on devices running iOS 15.0 or later.
iPad Pro, iPadOS 17