I don't really understand why you've written the SelectionChange code the way you have. Target represents the cell that has just been selected - you shouldn't try to change that. I suggest you use:
[vba]Private Sub Worksheet_SelectionChange(ByVal Target As Range)

Dim rMonitor As Range
Set rMonitor = Me.Range("C218")

'ERROR is here
If Not Intersect(rMonitor, Target) Is Nothing Then


Signature



End If
End Sub

[/vba]