I would like to buy Apple Care for my

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Buy an AppleCare plan - Apple Support

If you are within the 60-day “window” of opportunity to add AppleCare+, you will be able to do so directly from your iPad Settings - and it will be obvious that you can do so.

However, if you can’t see the invitation to add this cover, in iPad Settings, then your device is [no longer] eligible for this cover.