You typically need to enter a credit card or other valid method of payment* even if you don't plan on needing to use it. The document 'If you can't remove your last payment method or use no payment method with your Apple ID' ( https://support.apple.com/HT203905 ) mentions, "if you're using an existing Apple ID with the App Store for the first time, you must provide payment information." Note an extension of this: "Some purchases might require that you have a credit card on file, even if you use your Apple ID balance to make the purchase." - from: What you can buy with your Apple ID balance --> https://support.apple.com/HT211187
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:
On a mobile device tap: Settings > [your name at the top] > Media & Purchases > View Account > Review Apple ID.
Alternatively this document explains how to 'Change, add, [review,] or remove Apple ID payment methods' on an Apple mobile device, a Mac, or a PC; click here --> https://support.apple.com/HT201266
More things to try in this 2018 post by Urquhart1244 marked Apple Recommended:
https://discussions.apple.com/thread/8434425?answerId=33542414022#33542414022
If you continue to have problems contact Official Apple Support - Select your region from the list and then your country from the pop-up window. If you do not see your country, see if there is an "other" listing for that region.
https://getsupport.apple.com/?caller=cups&PGF=PGF63005
*Payment methods that you can use with your Apple ID - https://support.apple.com/HT202631