This is an error message returned from Authorize.net. It generally means you have the wrong username/password details entered in the Merchant Facility Tab in Salesforce. Or, you may have the Environment on the Merchant Facility incorrectly set (Production/Sandbox).
Please note, the "Sandbox" environment is used with Authorize.net accounts located at: https://sandbox.authorize.net/ If you are using a production account, but in "Test Mode", then this will most likely require the environment to still be set to Production. When "Test Mode" is enabled, authorize.net will only allow test cards to be used.
If using https://sandbox.authorize.net/ and environment of Sandbox, please make sure "Test Mode" is also set in the Authorize.net portal.
