this is not the Line Item 1 is invalid message, this is reporting a response code of 3 and a response reason code of 103:
3::1::103::This transaction cannot be accepted.
The authorize.net documatation:
Response_Reason_Codes_and_Response_Reason_Text.htm
says the response reason code 103 mans the transaction key is not correct.
"A valid fingerprint, Transaction Key, or password is required for this transaction."
if you are sure the transaction key is valid, you should contact Authorize.net to see why this is happening.
