You can’t participate in the Apple app ecosystem without two things: an Apple ID, and a valid credit/debit card linked to that ID. That’s right: you can’t get ‘free’ without having credit first.
If you’re setting up a new phone and your Apple ID is linked to another Apple ID via ‘Family Sharing’, you’ll also need the Family Sharing account owner’s credit/debit card expiry date and CCV number.
That’s right: your ability to use your phone with your credit card may depend on you having someone else’s credit card details.
Lesson learned: if your AppleID is part of “Family Sharing” and you set up a new phone, you will also have to confirm the payment details for the person whose credit/debit card details are linked to the “Family Group”. If that’s you, you’ll never notice this issue.
If that person’s not you, that might explain why you keep adding your credit/debit card details correctly and the system keeps complaining that your details aren’t complete.