Apple has multiple criteria* requiring the presence of a payment method, even for free items. Since you may be encountering one of those situations, you have no option but to enter a valid payment method if you wish to continue downloading. Read the support article Add a payment method to your Apple ID - Apple Support about how to do that. The payment method may not be charged if you also have account balance or the item is free, but the payment method has to be present. This next article lists valid payment methods for different countries but note that in this situation Apple Account balance alone apparently may not meet the requirement and you need to use another payment method until you find one that satifies Apple's requirement (often a credit or debit card): Payment methods that you can use with your Apple ID - Apple Support .
* Ref.:
If you can’t download or update apps on your iPhone or iPad - Apple Support
- "You might need to have a payment method on file, even if the app that you want to download is free."
Remove a payment method from your Apple ID - Apple Support
- If you have any active subscriptions, you must have at least one payment method on file other than Apple ID credit.
- You might not be able to remove a payment method if you have a balance due.
Sign in with your Apple ID - Apple Support
- "When you sign in to the App Store and other media services for the first time, you might need to add a credit card or other payment information."
How apps, content, and subscriptions from Apple are billed - Apple Support
- 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.
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."
If the amount being "charged" is very small it may be: How apps, content, and subscriptions from Apple are billed - Apple Support
"If you see a pending charge for a small amount
If you update your billing information or add someone to your Family Sharing group, a temporary authorization hold for a small amount might appear on your statement. Authorization holds are removed by your card issuer or financial institution after a short time."
- ~$1 to verify card, later removed.