"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
For a variety of reasons Apple will almost always require a functional payment method such as a credit or debit card be present when downloading anything from the iTunes or App Stores that isn't totally free, even if the item will be paid for with Apple Account balance. The only solution to satisfy this requirement is to add a valid payment method even if you don't plan on needing to use 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:
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