Update from OP:
So here is the actual response from Apple during Merchant Validation:
{
"statusMessage": "Payment Services Exception merchantId=9..9 unauthorized to process transactions on behalf of merchantId=9..9 reason=\"9..9 never authorized mass enablement transactions to occur via 9..9\"",
"statusCode": "400"
}
How do we get authorized for mass enablement?
pgrennin
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We believe the issue was caused by our macOS version not being the most up to date version. We successfully tested adding Amex cards to an iPhone 6 wallet with most recent software update. We are using macOS Mojave version 10.14.6. So iPhone 6 with up to date iOS can add Amex card, but Macbook with old macOS cannot add Amex cards. Suggest trying to update macOS or iOS software for those facing similar issue.
I'm dealing with same issue. Amex sandbox cards not working. Did anybody here ever figure it out?