This looks like the full request, you will also need the full response. Here is the code you can add to the page to get back both of these:
<?php
require_once("WA_eCart/Adv_CO_Scripts/AuthNet_PHP.php");
?>
<?php
if(!session_id()) session_start();
echo("the full request is: " . AuthNet_Result("Full Request") . "<br/><br/>");
echo("the full response is: " . AuthNet_Result("Full Response"));
?>
Please post back with the full response so we can see what the error message is.