How to verify payment to use Apple ID balance (gift cards) for in App purchases/upgrades

I have redeemed several Apple Store gift cards on my iPad. It shows the balance. When trying to purchase a subscription or in app purchase/upgrade, it goes through but then says before purchase can be completed I must verify the purchase so it takes me to the screen where you choose your payment method. This shows the balance of the giftcards in my Apple ID. The only choices on this screen are: redeem (to add more gift cards) or add payment method (to add other forms of payment.) How do I use Apple ID balance to pay for these purchases?