What is the rated, internal data storage space/capacity of your iPad model and how much of this free, internal data storage capacity is left inside of your iPad model?
All of that additional info can, also, be found in the Settings app, General settings panel, About.
Your iPad needs sufficient free internal data storage space. At the very least, 7-8 GBs or greater internal, free data storage space.
Is your iPad using/running a VPN? You need to, temporarily, disable the VPN.
Is your iPad running an iPadOS Beta version? You need to remove/uninstall the iPadOS Beta software before you will be able to install the Apple “officially” released version of an iPadOS upgrade/update.
Also, make sure your iPad is charged to, at least, 50%, or greater, before attempting the iPadOS upgrade/update.