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.
Is your iPad using a VPN? You need to disable the VPN.
Is your iPad running an iPadOS Beta version? You need to remove/uninstall the iPadOS Beta software.
Also, make sure your iPad is charged to, at least, 50% or greater before using the computer to do the iPadOS upgrade/update.
Make sure your desktop/laptop computer has plenty of internal drive storage space, also.
Also, make sure to do a backup of your iPad's data, prior to upgrading/updating.
Backup to Apple iCloud and/or a full Mac or Windows PC using the same iTunes app OR, if a newer Mac running macOS Catalina, or later, making a backup using the macOS Finder app.
How to back up your iPhone, iPad, and iPod touch - Apple Support