Add code in the On Current event procedure for the form to test the value of the control in question. If it's not null then display a message, cancel the event, and take the user to the control that must be filled in.
Private Sub Form_Current()
If Not IsNull(Me.Control1) Then
If IsNull(Me.Control2) Then
MsgBox "You must enter a value for Lot #", vbInformation
DoCmd.CancelEvent
DoCmd.GoToControl "Control2"
Exit Sub
End If
End If
End Sub