How to get a text to display on vba when a button is pressed?

 Sum = Application.Sum(Range(Cells(12, 3), Cells(12, 82)))

         If Sum = 0 Then

        Enter = "Enter a value for  E"


         Exit Sub

         End If

So I am trying to get "Enter a value for E to pop up when the button is clicked. So far when the statement is true, nothing happens.