Have you set it at a Table level?

Similar concept: Duplicate Values on Multiple Fields

Otherwise DLOOKUP on a BeforeUpdate

All I would like is the VBA code for this proceedure that I could copy and paste into the field that I want to use it in. I am unable to set at Table level for some reason that I don't understand. Thanks.

This would be much simpler to setup on table level with indexing. What's it saying that it wont allow that?

If you want to try some kind of VBA code though, I think you could try something like this using your field names. Up to you if you keep the Me.Undo in there

    If SerialNumber = DLookup("SerialNumber", "TransmissionT", "SerialNumber=" & SerialNumber) Then
        Me.Undo
        MsgBox "Error, Duplicate"
    Else
        'not duplicate
    End If

Or I think you could Dim a variable and use DCount as well - https://599cd.com/tips/access/dcount/

Then say something like this, just some ideas

If X <> 0 Then
'Duplicate
Else
'Not Duplicate
End If

I agree with Adam. I can't think of a reason why you couldn't set this up at the table level. Indexed (No Duplicates).