Google lemme down hard and you're my only hope.

I am trying to restore my girlfriends old gen 1 iPad that hasn't turned on in many many years but when I connect it to my MacBook and try I get ..

"The iPad “iPad” could not be restored. An unknown error occurred (21)."

My best guess is tat it can't update due to its last OS being so outdated so what is the solution? I can't be the only one with this issue.