Require Data To Be Entered In Cell Unless Selecting Specific Cells

I came across a problem with my workbook where the best solution I can think of would be this:

I need to prevent data from being entered anywhere else on a worksheet unless Cell X1 receives data first - anywhere else, that is, except for Cells U6, O79, U86, O159, U166, and O239.

How do I edit my existing code to do this?

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    Application.EnableAutoComplete = IIf(Not Intersect(Range("A9:A76,A89:A156,A169:A236"), Target) Is Nothing, False, True)
    With Range("X1")
        If .Value = Empty Then
            Application.EnableEvents = False
            .Select
            MsgBox "Please enter the number of pages required before entering data elsewhere."
            Application.EnableEvents = True
        End If
    End With
End Sub