Checking out with Authorize.net results in error 123 - This account has not been given the permission(s) required for this request

It is likely you entered the wrong Merchant ID in the Merchant Setup.

Merchant ID's are also known as merchant API login ID. This is different from the login ID you use when you login to manage your account at Authorize.net. It's usually something that consists of letters and numbers and it's not something you choose yourself. If unsure, please check with your Merchant Account Provider.