It wants to see you have a credit card or other valid method of payment even if you don't plan on needing to use it. There is no single, clear statement from Apple why this is required and speculation is not allowed on the forum.
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