This Error is from the payment gateway and is shown below:
This error could mean:
Server was unable to process request - Unable to process update. Add payment denied. This customer is on hold due to invalid credit card details and these card details have not been changed. A hold has been placed on this account. Please reach out to Ezidebit to clear the hold.
Unable to process update - Invalid Client Product for client ### - ### - ### and card type MASTERCARD (Wsvc)
This error happens when real-time payments settings for a customer site is turned on but they do not have real-time payments with EziDebit.
This is noted as an error code under Ezidebit documentation here
For this check that you do have a test EziDebit Account and you've entered the correct username/password details. This is as opposed to using the demo one and changing the payment gateway from the Windcave test to EziDebit test. Or, even using the live production one, but changing the environment form Production to Sandbox on the merchant facility.