I can’t download apps

Apple requiring a payment method such as a credit or debit card, even for free apps or where you have enough account balance to pay for the item, is not an uncommon observation. Some possible reasons are outlined in the links below such as outstanding balance, an issue with a payment method you have currently entered, ongoing subscriptions, or simply because they say so. You can try to configure your account to not have a payment method but you may find there is no option other than to enter a valid payment method to satisfy one of Apple's requirements. If the item is free you will not be charged. If it costs something, your Apple ID balance will in most cases be used first. You can later try to remove the payment method and see if it only needs to be there temporarily.

If you can’t download or update apps on your iPhone or iPad - 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." If you have something else on your account that requires a payment method, such as a subscription, and your current payment method is not working then Apple won't let you download even free things until you fix the payment method. This is explained in this document, 'If your payment method is declined in the App Store or iTunes Store' - If your payment method is declined in the App Store or iTunes Store - Apple Support