Turn on Debug-Log-To-File (or to email) in that module, and then do a test transaction. Then post the log file's contents here.
Also, contact your Authnet account representative and confirm what currency your account is configured for. They recently added support for additional currencies (for different countries), and if your account isn't configured for the currency you're submitting, you'll get errors. If your rep says it's configured for something other than USD, then some coding changes will be required to accommodate that: https://github.com/zencart/zencart/c...5c0319a9933be7