Question : Why is there a limitation that only one clustered index exists per table

Why should there be only one clustered index for  a table in SQL SERVER?

Answer : Why is there a limitation that only one clustered index exists per table

You either need a Handles BTNCheckPrevious.Command at the end of the sub, or an OnCommand="BTNCheckPrevious_Command" in the HTML.  Since this is not a top-level command, I would go with the OnCommand in the HTML.