This error indicates that the API Login ID or Transaction Key you have entered into the Payment Processing section of your site is incorrect. To resolve this issue:
- Login to your Authorize.net account and click on ‘Settings’.
- Select ‘API Login ID, and Transaction Key’
- Copy the API Login ID found here, and generate a new Transaction Key.
- Login to your site and re-enter the API Login ID, and Transaction Key, under ‘Edit Store’ -> ‘Shopping Cart’ -> ‘Payment Processing’.