Why won’t my iPad backup?
Make sure that iCloud backup is turned on.
(Settings -> tap on your name at top -> tap on iCloud -> tap on iCloud backup -> turn on.

iPad will now automatically back up daily when the screen is locked, wifi is on, and iPad plugged into power.

You can also backup manually from this screen by tapping backup now to iCloud.

If you don't have enough storage available on iCloud you will get an error message.

You can also backup to a computer by connecting through usb, then "trusting" the iPad (find the iPad in Finder) then the option to backup to computer will become available. Or use iTunes.