Help: Macro NOT Working After Protecting Sheet

Hello,

I have a workbook that I don't want the users to be able to change anything of the format, columns, rows, etc. so I protected the worksheet.
I have a macro saved in the sheet itself and not as a module that would allow users to select more than one option in the data validation drop-down lists. When I protect the worksheet I am not able to select more than one entry from the drop-down list. Hence, the macro is not working.

See attached workbook for more information

My question is: How can I protect the worksheet and at the same time have the macro work so I can select more than one option.

thanks for your help

below is the code that allows multiple selection

Option Explicit
' Developed by Contextures Inc.
' www.contextures.com
Private Sub Worksheet_Change(ByVal Target As Range)
Dim rngDV As Range
Dim oldVal As String
Dim newVal As String
Dim lUsed As Long

On Error Resume Next
Set rngDV = Cells.SpecialCells(xlCellTypeAllValidation)
On Error GoTo exitHandler

If rngDV Is Nothing Then GoTo exitHandler

If Intersect(Target, rngDV) Is Nothing Then
   'do nothing
Else
  Application.EnableEvents = False
  newVal = Target.Value
  Application.Undo
  oldVal = Target.Value
  Target.Value = newVal
  If Target.Column = 13 Or 14 Then
    If oldVal = "" Then
      'do nothing
      Else
      If newVal = "" Then
      'do nothing
      Else
        lUsed = InStr(1, oldVal, newVal)
        If lUsed > 0 Then
            If Right(oldVal, Len(newVal)) = newVal Then
                Target.Value = Left(oldVal, Len(oldVal) - Len(newVal) - 2)
            Else
                Target.Value = Replace(oldVal, newVal & ", ", "")
            End If
        Else
            Target.Value = oldVal _
              & ", " & newVal
        End If
        
      End If
    End If
  End If
End If

exitHandler:
  Application.EnableEvents = True

End Sub