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We are implementing Apple Pay on the Web. We are unable to add Amex test cards from the Sandbox from MacOS or iPhone wallet. We are attempting to add the Amex cards from (https://developer.apple.com/apple-pay/sandbox-testing/). Other card types (VISA, MC) can be added without issue. In Wallet & Apple Pay settings when I attempt to add the Amex card it shows "Authenticating..." near the device account number indefinitely, and it never gets activated properly.I've tried restarting my computer and ensuring device region is in US. I'm running macOS 10.14.6 (and due to security can't upgrade the OS). Can you confirm whether the OS is cause of this or something else with Amex cards not being supported.STEPS TO REPRODUCEOn Mac, go to System Preference > Wallet.Try to add any of the Amex cards from Sandbox here:https://developer.apple.com/apple-pay/sandbox-testing/The card initially shows as added but when you look at device account number it says "Activating..." indefinitely and is never activated.

My company has implemented Apple Pay on the Web. We are getting 400 and 403 status codes returned from Apple during the merchant validation process and have no idea why. All the required fields described here are provided. (https://developer.apple.com/documentation/apple_pay_on_the_web/apple_pay_js_api/requesting_an_apple_pay_payment_session) We completed the configuration of environments, registering and verifying domains, and have even tested successfully on lower environments.