This is the error from Authorize.net. So we know that you are reaching Authorize.net but there is an issue with the credentials you sent. Please make sure you have entered valid credentials in your Forms>Settings>CIM page. That is the main Settings page under Forms – Not the feed settings under each individual form. (I know – it’s confusing. But that is how Gravity Forms is set up and it works great once you’re set up. :/) Here is an example from the CIM plugin but they all look similar.

If you’re having trouble after entering, please visit Authorize.net support for answers about Transaction Key generation. (FYI Sometimes people inadvertently enter their login information for their A.net account. That will not work. You need an API Login ID and a generated super secret Transaction Key.)