This error could be caused by several things, but most commonly:

1) Your gateway login or key are incorrect. Please ensure that you are using the API credentials and NOT your login credentials that you use to log into Authorize.Net's control panel. Please use Webfill again and ensure that there is no extra space when you click submit. This fixes the issue for 99% of people. We realize it's a tricky thing to do, but your patience and perseverance will pay off!

2) Your gateway account may be set up incorrectly. Please contact your merchant account provider (Applied Merchant?) so that can look into your account and see if there's a problem. Sometimes accounts are set up to accept "Card Present" transactions only, and this will cause this error.