It wants to see you have a credit card or other valid method of payment even if you don't plan on using it.
If there is an option to select "Review" or "Continue" , select that and enter valid payment information if required. If there's problems doing this:
Tap Settings > [your name at the top] > Media & Purchases > View Account > Review Apple ID.
More things to try in this 2018 post by Urquhart1244 marked Apple Recommended:
https://discussions.apple.com/thread/8434425?answerId=33542414022#33542414022
Payment methods that you can use with your Apple ID - https://support.apple.com/HT202631