Error 424 In Macro

I am a very basic user in Excel and I piece mealed together a macro that takes a row and copies the values to a new sheet if a cell in that row meets a particular criteria. I then go to delete that row from the original sheet. The row gets deleted but I get an 424 Error.

Sub Sort()
Dim LastRowMain As Long
Dim LastRowNewOrd As Long
Dim i As Long
Application.ScreenUpdating = False
LastRowMain = Worksheets("Lettres").Range("A" & Rows.Count).End(xlUp).Row
With Worksheets("Lettres")
For i = 2 To LastRowMain Step 1
Select Case Cells(i, "M").Value
' This copies Oui to Fermer
Case "Oui"................................