This code should do what you want

Also as a note for future reference don’t use spaces in any table name
form name field in a table anything as it creates problems when
programming use camel case like this CopyOfDIV3ICTDatabase

You forgot to put [] around Copy of DIV 3 ICT Database because of the
spaces in your code

Private Sub Case_Number_BeforeUpdate(Cancel As Integer)
If isnull(Dlookup("[Case Number]","[Copy of DIV 3 ICT Database]",
"[Case Number] = '" & [Case
Number].value)) Then
DoCmd.GoToRecord , , acNewRec
else
If isnull(Dlookup("[Date Completed]","[Copy of DIV 3 ICT Database]",
"[Case Number] = '" & [Case
Number].value)) Then
Me.Filter = "id = 2"
Me.FilterOn = True
Me.Requery
else
Cancel = True
Me.Undo
End If
end if
End Sub

Hope this helps

Regards
Kelvan