If Cell Value = Error, show MsgBox

May be this....

Private Sub Worksheet_Calculate()

If IsError(Range("Z18").Value) Then
    If MsgBox("If MsgBox(Reference not found! Do you want to add it to database?", _
    vbYesNo + vbQuestion, "Update file to Management") = vbNo Then Exit Sub

    UserForm1.Show
End If

End Sub