No iCloud backups are compatible with the version of iOS 9.3.5 on this iPad. What do I do?

You are trying to restore an older backup with a newer iOS update. That will not work. The iPad you are trying to restore to is only compatible with iOS 9.3.5 which makes it quite an old iPad. An iPad compatible with iOS 9.3.5 has to be either an iPad 2nd or 3rd generation or an iPad Mini 1st generation. It appears that you may have created a backup on a much newer iPad with iPadOS 15.1 It is impossible to use those newer backups.