"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.
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
If you do not have a card, get a parent to add one of theirs. If they are worried about that then have them set up Family Sharing with Ask To Buy.
To learn how to 'Set up Family Sharing', click here --> https://support.apple.com/HT201088
Approve what kids buy with Ask to Buy - https://support.apple.com/HT201089 - Family Organizer receives requests by default, but another parent or guardian can be assigned permission.