ERROR_1 => Code: 91577 Message: Merchant account does not support payment instrument.
ERROR_2 => Code: 81703 Message: Credit card type is not accepted by this merchant account.
Seeing an error similar to the ones shown above means that your WHMCS system is attempting to process a card type that is not accepted by your Braintree account. For example, a customer pays with an AMEX card, but your Braintree account only allows VISA and Discover.
To fix this, you will need to contact Braintree to discuss adding other card types to your account. You should also only select the card types allowed by your Braintree account in your WHMCS installation’s “Invoice” tab under the “General Settings” option. The line item is titled “Accepted Credit Card Types”.