Apple Pay card "Not available for Web Payments" and "Not enabled for payments on the web"

I'm trying to check out from the [Apple Store](https://www.apple.com/store) on Safari for macOS using Apple Pay. In the Apple Pay credit card selector, the card I want to use cannot be selected. It's noted as: > Not available for Web Payments: > > (Not enabled for payments on the web) The card I want to use but is "not available" is an American Express. The cards that are available are both VISA, one debit card and one credit card. Why can I cannot select the AMEX? How can I enable it for payments on the web?