https://developer.apple.com/support/apple-pay-sandbox/ has a bunch of test cards you can use for sandbox testing. I was able to add a sandbox Visa, MasterCard, and Discover card successfully to my sandbox test account. But any time I try to add an American Express card (from the list), I get "Could Not Add Card" "Try again later or contact your card issuer for more information." Anyone else encountered this?
I am getting the same with all Amex cards... disappointing
Did you create a sandbox user account, as you might not be able to add test credit cards on your account which is not a sandbox account
Please fix being able to add Amex cards for Apple Pay testing.
6 years later and this is still an issue... it seems Apple is dragging their feet on Apple Pay sandbox support.