Payment
How to enter a new credit card number when old card lost/stolen and there is an outstanding payment due. Editing and adding new card doesn’t work.
iPhone 6 Plus, iOS 12
Question marked as
If you can't remove your last payment method or use no payment method with your Apple ID
https://support.apple.com/en-ca/HT203905
Pay an unpaid balance
You can't remove a payment method if you have a balance due. Learn how to pay an unpaid balance or fix payment method issues.
After the unpaid balance is paid, you can remove your payment method.