First, are you sure it isn't just asking you to have a payment method on record? Also noteh part below about not being able to buy some thing with account balance.
This document outlines what you can buy with your Apple ID [Apple Account] balance --> What you can buy with your Apple Gift Card or Apple Account balance - Apple Support
"Some purchases might require that you have a credit card on file, even if you use your Apple Account balance to make the purchase."
How App Store and iTunes Store purchases are billed - How apps, content, and subscriptions from Apple are billed - Apple Support
Some purchases aren’t charged to your Apple ID balance
- You can’t charge gifts to your Apple ID balance. If you send a gift or gift card, your primary payment method is charged.
- Some subscriptions might not be charged to your Apple ID balance.