Do following change in your code..
<asp:gridview id="grdViewManualAttendance" runat="server" autogeneratecolumns="False" backcolor="White" bordercolor="#999999" borderstyle="None" borderwidth="1px" enableviewstate="False" font-size="Small" width="680px" xmlns:asp="#unknown">OnRowCommand="Griddealers_RowCommand"
<asp:templatefield><HeaderTemplate><asp:button id="btnDelete" runat="server" text="Delete" onclick="Clickevent()">
</asp:button></asp:templatefield></asp:gridview>