iPad backup issue

See the following two links for help with backup issues, depending on where you are trying to backup:

If trying to backup to your computer --> If you can't back up your iOS or iPadOS device to a computer or restore it from a backup - Apple Support

If trying to backup to iCloud --> If you can't back up to iCloud - Apple Support

The most likely reason is that your backup requires more storage than you have in iCloud. You can edit what you’re backing up and remove some data.