What causes the error "The supplied variables are not according to specification"?

This error is thrown if any of the POSTed values are incorrect, for instance, if the merchant_id or merchant_key is incorrect (possibly due to using a sandbox credentials on the live site), the following error will be thrown ‘The supplied variables are not according to specification: merchant_key: Merchant key is invalid’. The error will highlight which variables are incorrect or missing, you will then need to ensure that the POST form is adjusted accordingly.