iCloud payment

Settings, Your name, Payments & Shipping

Try to add a card there.

Reasons your payment card may be declined

1.            Account suspended for suspected fraud.

2.            Address in your iTunes account does not match exactly the billing address for the card

3.            Error in credit card number

4.            Insufficient balance or you still owe money on a subscription

5.            The card was not issued in the country of the iTunes store

6.            Wrong expiration date

7.            Wrong security (CVV) code

8.            You are connected through a VPN provider that makes iTunes think you live elsewhere - uninstall VPN

Here is additional information from Apple if your payment method is declined.

If your payment method is declined in the App Store or iTunes Store - Apple Support