Answer
This message comes from the employee's super fund and is passed back to you through Ozedi. It means the fund received a SuperStream message from Lightning Payroll promising a payment with a particular reference, but no payment carrying that reference ever arrived.
In almost every case it is left over from a correction rather than a real problem. A SuperStream message was sent but never paid, either because payment was overlooked or because something needed changing first. The message was sent again later, and only that second message was paid. The first message is still sitting with the fund waiting on money that was never sent under that reference, so the fund reports it as unmatched.
How to tell which message it refers to
Open the response PDF attached to the email. The upload date shown at the top tells you when the message being complained about was originally sent. If that date is well before the payment you actually made, you are looking at an earlier superseded attempt.
You can then go to Super Fund Deposits >> SuperStream Mailbox and set the date range at the top of the screen to cover that upload date. Open the message to read the fund's full response. The ABA Reference shown at the top of the message is the payment reference the fund was told to expect.
When you can ignore it
If a later message for the same employee and period was sent and paid, the contribution has already reached the fund and this earlier unmatched message can be ignored. You may receive this same response more than once for the one message. It does not need any action.
The employee's fund balance is the final word, so if you want certainty, ask the employee to confirm the amount was credited to their account.
When it does need action
Treat it as a genuine failure in two situations: this is the only message you sent for that employee and pay period, or the money has been returned to your bank account. In both cases the contribution has not reached the fund and must be sent again.
To resend, use Manually Fail on the original message to release its deposits, then send those deposits as a new message by following How Do I Send Existing Super Fund Deposits?
Please note: some banks do not pass on the full payment reference when you type a transfer in by hand, or they shorten it. If your reference matched exactly and the money was not returned, the mismatch sits with the bank or the fund rather than with Lightning Payroll, and is worth raising with them directly. Paying with the ABA file Lightning Payroll creates avoids this, because the correct reference is written into the file for you.