By default all cells are blocked. So first unblock them by selecting all cells (Ctrl+a or click on the spot above the 1 of row 1) and going to cell properties (Ctrl+1 or right-click > cell properties).
Go to the final tab called protection and uncheck the blocked option.
Now select A2:M2 and check the blocked option again.
Once that is done, implement the following code (right-click sheets tab > view code > paste code) and try it out:
Private Sub Worksheet_Change(ByVal Target As Range)
If Intersect(Target, Range("M2")) Is Nothing Then Exit Sub
If Target = "app" Then
ActiveSheet.Protect Password:="Password", DrawingObjects:=True, Contents:=True, Scenarios:=True
Monday, Tuesday and Thursday are usually the days I'll respond. Bear this in mind when awaiting a reply.