lock/unlock cell based on another cells input

I tried DonkeyOte's idea and it worked but not quite like I wanted it to.

When I tried royUK's code I come up with a compile error: Ambiguous name detected: Worksheet_SelectionChange

here is the code that I have

[Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    Const PW   As String = "secret"
    If Target.Address = "$G$12" Then
        Select Case Cells(7, 8).Value
            Case "Grass Fire", "Timber Fire"
                ActiveSheet.Unprotect PW
                Target.Locked = False
                ActiveSheet.Protect PW
        End Select
        End If
End Sub]