My iPad won't update even when it has enough space

It is a little hard to help/advise you when you provide little to NO useful information about your iPad.

Lots of iPad year and models.

Here are my questions to you.

Can you, please, confirm what year and model iPad do you have/own?

The “A” model number can be found on the aluminium back of your iPad, in very small print, right under the larger bold print “iPad” on the back of the iPad.

AND/OR

Supply the “M” model number, which can be found in the Settings app, under the General settings panel, then tap About.

What iOS/iPadOS version is this iPad, currently, running? Your iOS/iPadOS version maybe too far behind in iOS/iPadOS versions OR cannot be upgraded/updates any farther than what your iPad OS version is already, currently, running.

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.