iPad Pro Update

The sixth generation iPad Pro launched with iPadOS16. Rolling-back to an earlier version of iOS/iPadOS, such as iPadOS15.x, is not possible.

All System Updates are digitally signed by Apple - and can only be installed whilst the digital signatures remain valid. After release of a new iOS/iPadOS version, the immediately preceding version continues to be digitally signed by Apple - however, usually within a few days, Apple will revoke the digital certificate with which the earlier version is signed.

A 6th generation iPad Pro would have been released with iOS 16, so you may have to provide further details.