How to download apps

Follow the steps here: Change, add, or remove Apple ID payment methods - Apple Support

Note payment and billing address must be for rthe country the Apple ID (store) is set to.

You cannot add a payment method with a billing address in acountry that is different to the one set for the Apple ID.

See here to verify the Country or Region set for your Apple ID: Change your Apple ID country or region - Apple Support

Well you need to sort out your billing issue before you can download

You need to fix out the issue it is flagging.

There will be a reason why

Maybe contact apple