Hi Stephanola84,
Thanks for reaching out using Apple Support Communities. Here to help.
We see you are wondering why a card you have registered on Apple Pay shows as Not available for in-app purchases.
This article lists the approved payment methods available, and pre-paid cards are not listed: Payment methods that you can use with your Apple ID - Apple Support
Some payment methods may be enabled for contactless payment but not virtual payment as would be the case with online purchases. This seems to be the case because the Serve card is a prepaid card that does show can be used for contactless payments. If you wish to ask if there is some option they can adjust to allow this, you will need to ask American Express.
"If you have trouble adding a payment method for purchases in the App Store, iTunes Store, or Apple Books, you might need to contact your financial institution for more help."
Hope this helps! Take care for now.