Hi
Apple Pay - Apple's payment service for paying in stores, on websites and within apps - cannot use a card that you have not specifically added to Apple Pay.
You appear to instead be referring to your Apple ID account payment method, which is used when making purchases from the iTunes Store, App Store and for in-app purchases.
To check and, if desired, change your Apple ID payment method, follow the instructions here:
Change or remove your Apple ID payment information - Apple Support
More information about Apple Pay, which is a different service, is available here: