You don't Mention how G24 Becomes the Word "pop"
If you are manually change the cell then try

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$G$24" Then
If Target = "pop" Then Msg = MsgBox("Both Values do not
agree", vbOKOnly)
End If
End Sub

If the cell changes by calculation then try this

Private Sub Worksheet_Calculate()
Dim r As Range
Set r = Range("G24")
If r = "pop" Then Msg = MsgBox("Both Values do not agree",
vbOKOnly)

End Sub