While adding an Amex card to the sandbox tester account, it is showing an error as "This card cannot be used".
The card initially shows as added but when you look at the device account number it says "Activating..." indefinitely and is never activated but MC, VISA cards are added successfully.
Following probable solutions have been tried: Setting device location to the US
Creating a new sandbox account and tried adding but no luck.
Note: Tried with all Amex Cards mentioned in https://developer.apple.com/apple-pay/sandbox-testing/
apcaseys
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution