Payment method requested even though one already exists.

You could test the app by trying to make a purchase in another app or in the App Store. That would be the first step.

Great!

If you want to go deeper to know what happened, you can contact Apple using The Support app in any of your iPhones, if it isn't installed already, do so and after you enter into the app, scroll almost all the way down you'll find "More Products>" and App Store, select that one and then Subscriptions & Purchases, Update payment method, and after you'll get a message scroll back again to the end and select "Get more help" and there ask to chat with an Apple Support representative, there can track down the problem, to avoid further complications if the issue presents again with any other purchase. Hope it helps!

The App Store checks for the payment method even on free or any cost charges; when this has happened to me, is usually because the card or payment method has no funds. By adding some real money has help me to complete the transaction even though there’s not charge made. Hope this help.