iPadOS 14.8/14.7 cannot upgrade to IOS 15

Your screenshot illustrates that your iPad is centrally managed - presumably by your employer, Corporate or Educational establishment. The applied device-management policy restricts your ability to update your iPad.

You will need to seek assistance from the System Administrator who manages the device.

Did you ever install beta versions of iOS 14.x? IF YES, then you may still have the profile installed on your iPad. Delete the profile in Settings ➔ General ➔ Device Management and restart your iPad. Once completed then you can attempt to update your iPad.

Axel F.