A 64 GBs storage iPad starts out with 60 GBs., after drive formatting.
Minus approx. 6 GBs for iOS/iPadOS, leaves 54 GBs.
Minus 2-4 GBs for iOS iPadOS system reserve, to ensure acceptable, nominal iOS operational performance, equals approx. between 50-52 GBs of actual, usable data storage space.
Your iPad ONLY has approx. 11 GBs left.
My guess is you simply do not have a sufficient amount of free, internal data storage left to do an over the air, wireless, WiFi upgrade of iPadOS.
You need to try using the latest iTunes on a Windows PC or older Mac or if a newer Mac with a newer macOS using the macOS Finder to upgrade that iPad.
This is your ONLY other option to upgrade that iPad.
Best of Luck to You!