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. Read the support article Add a payment method to your Apple ID - Apple Support about how to do that. 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. If you are being asked to enter a payment method, Apple has multiple criteria* requiring the presence of a payment method, even for free items.