Run Time error 1004

[Marcia]

Hi. I have a workbook with a macro that fills the active cells. When I locked all the formulae to prevent them from inadvertent edits, the "Run time error 1004" appears everytime I go to unlocked cells.
My knowledge of vba and macro is to google, copy, paste, and record so I am at a loss on how to remove the error.
Thank you for the usual quick help.

[p45cal]

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Me.Protect UserInterfaceOnly:=True, AllowFormattingCells:=True
Cells.FormatConditions.Delete
Target.FormatConditions.Add Type:=xlExpression, Formula1:="TRUE"
Target.FormatConditions(1).Interior.ColorIndex = 40
End Sub

[Marcia]

Perfect. Again, big big thanks p45Cal. Had you not come to the rescue, my last resort would be to delete the highlight code.