Apple will not / never charge you zero dollars ($0.00).
You need to have a valid payment method on file, even if the app you want to download is free.
Free apps has also In-app pruchase.
If you have set your Apple Account balance as your default payment method, your default payment method will be used first by Apple when you need to make a payment.
Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.
See the following Apple articles:
Add a payment method to your Apple ID
Add a payment method to your Apple ID – Apple Support (UK)
Add money to your Apple Account balance
Add money to your Apple Account balance - Apple Support
Payment methods you can use with your Apple ID
Payment methods you can use with your Apple ID – Apple Support (UK)
fyi: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card. If your card is not accepted as a paymenf method, you need to contact your bank.