VBA to unlock cells, run macro then re-lock them

Hi
I have the following code in a worksheet of which Ive locked columns AA & AB, then password protected the worksheet. However when I run my code I keep getting a

run-time error '1004': unable to set the locked property of the range class error

.
Can anybody help please....I am assuming I need some code to unlock the sheet with the password before unlocking and then re-locking the cells??

Private Sub Worksheet_Change(ByVal Target As Range)
    
    If Target.Column = 1 Or Target.Column = 2 Or Target.Column = 3 Or _
    Target.Column = 4 Or Target.Column = 5 Or Target.Column = 6 Or _
    Target.Column = 7 Or Target.Column = 8 Or Target.Column = 9 Or _
    Target.Column = 10 Or Target.Column = 11 Or Target.Column = 12 Or _
    Target.Column = 13 Or Target.Column = 14 Or Target.Column = 15 Or _
    Target.Column = 16 Or Target.Column = 17 Or Target.Column = 18 Then
        
        Cells(Target.Row, 27).Locked = False
        Cells(Target.Row, 28).Locked = False
        Cells(Target.Row, 27).Value = Now
        Cells(Target.Row, 28).Value = Environ("Username")
        Cells(Target.Row, 27).Locked = True
        Cells(Target.Row, 28).Locked = True
                
        End If
        
End Sub

Thanks in advance

Jen