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.
IF you have an iPadOS 15.7.1 or iPadOS 16.1, or later, compatible iPad, that iPad year and model is, probably, waaaay too far behind in iOS/iPadOS upgrades/updates.
If you have access to a full computer, like a Mac or Windows PC, try either using the Finder on a Mac that has the newest macOS Catalina, or later macOS, installed OR with an older Mac with an older Mac OS, download and install the latest iTunes version ( or latest iTunes for Windows PC ), make sure the Mac or Windows PC is connected to the Internet, connect your iPad by the USB charge cable to the computer and let the iTunes application upgrade/update that iPad model, using the iTunes Software Update feature, to its current, maximum OS of iOS 12.5.6/iPadOS 15.7.1/iPadOS 16.1.1, depending on your year and model of iPad.
A 2013 iPad Air 1st gen, iPad Mini 2nd gen and 2014 iPad Mini 3 can ONLY be updated to iOS 12.5.6.
A 2014 iPad Air 2 and a 2015 iPad Mini 4 can ONLY be upgraded to iPadOS 15.7.1
Any other newer iPad model from late 2015 can be upgraded to the latest iPadOS 16.1.1.
At this late stage, use of a Mac or Windows PC is, now, unavoidable.
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!