Seems for some transactions Apple requires a debit or credit card be listed in your Apple ID account.
This is even when money in an Apple ID Account is actually used for the payment / purchase.
(Seems the Apple account balance is always tried first, for every purchase / payment etc..)
 Apple Support have an article online (link below) that that may or may not help.
(We are all Apple users, like yourself, so often don't know why Apple does what it does .... to keep our money safe.)
What you can buy with your Apple Gift Card or Apple Account balance - Apple Support
How to: Change, add, or remove Apple ID payment methods - Apple Support
All the best :-)