Can't download apps due to payment method issue in Apple Account

Seems Apple requires a payment method is listed in your Apple Account for the first time you use the App Store, or iTunes, even if you only download free apps. (Seems Apple don't charge your payment method for free apps.)

You would have used an Apple Account to log-in here, so am assuming you do have one already/

(If you don't want to add a credit or debit card, in some countries you can create a balance with a gift card.)

See which payment methods Apple can accept in your country or region (you may need to scroll down):

Payment methods that you can use with your Apple Account - Apple Support

See how to: Add a payment method to your Apple Account - Apple Support