Cannot download iOS 14

you can try connecting it by cable to a computer and use itunes on the computer to update

Update your iPhone, iPad, or iPod touch - Apple Support

Perhaps because you have the iPadOS14 beta installed.

https://beta.apple.com/sp/betaprogram/unenroll#ipados

How to uninstall iOS beta software - Apple Support

If the production version of iPadOS14 is already installed on your iPad, what, precisely, is the problem?