The error message that you see clearly indicates that, for some reason, Apple's regional update server(s) are unavailable.
While this is unusual, this condition can occur from time to time - such as when servers are offline for maintenance, or when the servers are unable to simultaneously respond to all incoming update requests.
If you see this error, you have no alternative other than to wait and try again later. If the problem persists for more than 24 hours, you might be well advised to seek assistance directly from Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page.
Alternatively you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:
https://apps.apple.com/app/apple-support/id1130498044
You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to assist with updating your iPad to its most recent supported version of iPadOS - at the time of writing, this being iPadOS 26.2.1:
Genius Bar Reservation and Apple Support Options - Apple