Clear contents of column(L) after every one row

Try

Sub test()
    Dim n As Long
    With Range("k6", Range("k" & Rows.Count).End(xlUp)).Offset(, 1)
        n = .Row Mod 2
        .Value = .Parent.Evaluate("if(mod(row(" & .Address & "),2)<>" & n & _
            ",if(" & .Address & "<>""""," & .Address & ",""""),"""")")
    End With
End Sub