When using PayPal Website Payments Pro as your payment gateway, you may find that your visitors trying to pay you using an American Express (AMEX) card receive the following error message after submitting their billing information:
"Payment Error: (10566) - Credit card type unsupported"
PayPal may require their payment gateway customers (not your donors, supporters, etc) to separately agree to terms of service for taking payment via AMEX. To resolve this issue, contact PayPal directly and let them know of the error message received and that you're using PayPal Website Payments Pro as your payment gateway.
After PayPal resolves the issue with your account, you should be able to process AMEX via any of the Soapbox Engage apps.