My iPad won't let me update to iOS 16

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

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?

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.

Please try to make an attempt to answer all questions asked

Is your iPad using a VPN? Disable it.

Is your iPad using Beta iPadOS software? You will need to remove/unistall it.

Also, make sure your iPad’s battery is, at least, charged up to 50% or greater.