Cannot make in-app purchases using App Store & iTunes credits

I want to make purchases in a game that I have downloaded on my phone but whenever I do it automatically charges my credit card instead of using my gift card balance in my wallet . I even deleted all my subscriptions and my credit card information however it will still not allow me to use my credits. It says “payment information required”. After months of trying to figure it out, I am beyond frustrated!! What do I have to do to use my gift card balance!?