Unable to add AppleCare for iPad

A couple of things can stop AppleCare from being offered. First, check your iPad's current coverage at Settings > General > About > Coverage, or enter the serial number at https://checkcoverage.apple.com . If it already shows AppleCare or an expired window, that explains it. Second, AppleCare+ generally has to be added within a limited period after you buy the iPad (in the US that's 60 days; it varies by country), and it can't be added to a device bought used or refurbished from some resellers.

If you're within the window and it still won't let you add it, the Apple Support app or https://www.apple.com/applecare/ can sort it out with your serial number. Where do you get stuck: Settings, the Apple Store app, or the website? And roughly when did you buy the iPad?

Check the terms for purchasing AppleCare in the country where you live. Refer to this support article --> https://support.apple.com/104941

"In most countries and regions, you can buy AppleCare+ within 60 days of your device purchase. In Japan, you can buy AppleCare+ within 30 days of your device purchase. In some countries and regions, you can buy an AppleCare Protection Plan within 1 year of your purchase of an eligible Mac, Apple TV, or other device."

What does it say that indicates to you that you cannot buy AppleCare? You can check your eligibility as outlined in https://checkcoverage.apple.com/

If you are trying to buy it with account balance, you may need to augment payment with another method:

https://support.apple.com/118244

Apple Gift Card and Apple Account balance can't be used for some purchases:

• For some recurring charges, like AppleCare+, you can't start a subscription with Apple Account balance. But as long as you keep another valid payment method on file and your Apple Account balance would cover an entire charge, a recurring charge will pull from your Apple Account balance. In some countries and regions, Apple Account balance can’t be used for some subscriptions like AppleCare+.