This would be caused by an incorrect merchant ID and/or Key set on the PayFast configuration page on your Shopify site. You will need to ensure that the merchant ID and Key set on your Shopify store are identical to the merchant ID and key on your PayFast account and that no spaces have been adjoined to these values when copying them across.
When un-checking ‘Use Test Mode’ after testing the module, ensure that the merchant ID and key fields are populated correctly after doing so, as they may have been pre-populated with an email address by the browser. This is because Shopify treats the merchant ID and key fields as username and password fields, which are then auto-populated by some browsers.