How do I restore the computer backup from an older iPad to a new iPad
There's no general reason why you shouldn't be able to restore a backup of an iPad Air running iOS 12.5.1 on an 8th Gen iPad running iPadOS 14.4. In general, this should work without problems.
It could sound like the backup you have on the MacBook Pro is somehow corrupted.
I would advise taking a new full backup of the device, and trying the process over again.
Another way to go would be to use the &quot;Quick Start&quot; feature. Turn on your new (blank) iPad and hold it near the old iPad (ensure that Bluetooth is enabled). On the old iPad, you should see a popup asking if you want to setup your new iPad. When asked how to transfer data, select to transfer directly from the old iPad.
The Quick Start feature with direct data transfer is available in 12.4, so as you're running 12.5.1 you should be good to go.