Credentials Invalid data when preloading for Moneris Checkout

I'm having trouble with getting the preloading to work with MonerisCheckout. In PHP, I send a cURL request ( with the following body:
And I get this response:
{"response" : {"success":"false","error":{"field":"credentials","message":"invalid data"}}}

I did take the credentials from my developer account and from the test site for the MRC.


What does this error mean?