Downloading games from App Store help please

Hello I'm trying to downloads games from the App Store from my iphone. I have a $10 Apple ID balance that I'm trying to use because the debit card on file has nothing on it so it will not even let me get free games all it says is payment error so how can I switch it to use the apple is balance instead of the debit card from my iphone? No computer at the moment thank you in advance