Error: 91517/91577 Payment instrument is not accepted by this merchant account/merchant account does not support payment instrument. What's going on?

91517 and 91577 are validation errors. These errors indicate that a particular card type (American Express for example) or a token (read about tokens here associated with a particular card type isn't accepted by your merchant account. To resolve, you would need to reach out to your merchant account provider and enable the card type in question.