Can’t purchase Apple Care+ monthly plan

If you are within 60 days from the date of purchase of your iPad Pro, you should be able to do it using the instructions provided at the section "How to buy", at the very bottom of this support article:--> AppleCare Products - iPhone - Apple

For any further question about that, you can contact Apple Support directly, using this link:--> Contact Apple for support and service

Regards

Giulio

You should probably update your payment information, then:

1. Open Settings
2. Scroll down to find 'Wallet & Apple Pay' and tap it
3. Tap your payment card

If everything looks fine, I suggest contacting your bank. :)

AppleCare+ monthly payments are not available in all locations.

And what happened when you contacted Apple Support?

What did they tell you?