I can’t update my iPad air2

See if anything here helps.

Update your iOS device.        

Get help with over-the-air iOS updates          

Update – Can’t or can’t restore 

Have you tried using iTunes on a computer or Finder if running Catalina to do the update?

 Update software on iOS devices in iTunes on Mac

If you see an error when you update or restore your iPhone, iPad, or iPod      

Which version of iPadOS are you running now?

13.6.1 is current. If you’re on a version before 13.6.1, what happens when you try to update?

There’s a beta of the next release, but that’s only for folks with “spare” devices and no important data.