Make sure your iPad has no VPN running. If it does, temporarily, disable it.
Make sure your iPad is not running any test, Beta iOS/iPadOS software. If it is you'll need to remove/uninstall it.
The official Apple released OS upgrade/update version/s will not be present or be able to be installed whilst Apple OS Beta software installations are currently active and running.
Make sure your iPad is, at least, charged to 50% or greater to attempt this.
Also, make sure your iPad has, at least, between 6-8 GBs of free internal data storage space for downloading and installing a major iOS/iPadOS upgrade.
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
If NO access to a Mac or Windows computer, you will have to seek help from a good friend or family member, with a full Mac or Windows PC, to help you out
OR
Search online to locate and call contact your nearest, “local” Apple Store location to try/attempt to make an appointment to get your Apple iPad upgraded/updated to its latest iPadOS 15 version.
OR, alternatively, use the “Support” link, in the top black menu bar, at the upper right hand corner of every Apple Support Community Webpage to attempt to schedule an appointment with your nearest Apple Store location.
Apple Store employees will be glad to help/assist you.
Best of Luck to You!