update panel is not triggering

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C#5.0

My source code as follows

asp:UpdatePanel ID="Updatepanel1" runat="server">
<contenttemplate>

<ajaxToolkit:TabContainer ID="waitlistent" runat="server" CssClass="MyTabStyle" ActiveTabIndex = "0"
            OnActiveTabChanged="waitlistent_activetabchanged" AutoPostBack="true">
              <ajaxToolkit:TabPanel ID="waitlistentry" BackColor ="RosyBrown" runat="server">
           <HeaderTemplate>Waitlist Entry Screen
              </HeaderTemplate>

               <ContentTemplate>

<asp:Panel ID="entrypn" runat="server" BackColor="BurlyWood">
           <table style="width: 835px"><tr><td class="style1">

<asp:Label id="lblwecrs" Text="Course Name" runat="server" Width="32%" Font-Bold="True"
                    ForeColor="Blue">
                    </asp:Label>

</td>
                    <td style="height: 28px"><asp:DropDownList id="ddlcoursename" runat="server" Width="28%"
                           onselectedindexchanged="ddlcoursename_SelectedIndexChanged"
                           AutoPostBack="True"></asp:DropDownList><br /><asp:RequiredFieldValidator ID="RequiredFieldValidator3"
                    InitialValue="--Select Course--" runat="server" ControlToValidate="DdlCourseName"
                        ErrorMessage="Select the Course" Width="51%"></asp:RequiredFieldValidator></td></tr><tr><td class="style2"><br /><asp:Label id="Label5" Text="Non available dates" runat="server" Width="44%" Font-Bold="True"
                    ForeColor="Blue"></asp:Label></td><td style="height: 28px"><asp:DropDownList id="ddlbatchdate" runat="server" Width="28%"
                           AutoPostBack="True" onselectedindexchanged="ddlbatchdate_SelectedIndexChanged"></asp:DropDownList></td></tr><%--<tr>--%><td align = "center" class="style1"><br />&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; &#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;
                           <asp:Button ID="Btnentry" runat="server" OnClick="Btnentry_Click" Text="Submit" Height="24px" Width="88px" /></td><%-- </tr>--%></table></asp:Panel>



              </ContentTemplate>

           </ajaxToolkit:TabPanel>

<ajaxToolkit:TabPanel ID="waitlistconf" runat="server" BorderColor="Black" BackColor="Coral">
                 <HeaderTemplate>Waitlist Confirmation screen
                 </HeaderTemplate>

                    <ContentTemplate>

<asp:Panel ID="entryconf" runat="server" BackColor="BurlyWood">
                     <table style="width: 835px">
                      <tr>
                    <td style="width: 510px">
                    <asp:Label id="lblwccrs" Text="Course Name" runat="server" Width="32%" Font-Bold="True"
                    ForeColor="Blue">
                    </asp:Label>

</td>
<td style="height: 28px">
        <asp:DropDownList
            id="ddlcrsname" runat="server" Width="28%" AutoPostBack="True"
       onselectedindexchanged="ddlcrsname_SelectedIndexChanged">
       </asp:DropDownList><br /><asp:RequiredFieldValidator ID="RequiredFieldValidator1"
InitialValue="--Select Course--" runat="server" ControlToValidate="ddlcrsname"
    ErrorMessage="Select the Course" Width="155px"></asp:RequiredFieldValidator></td></tr><tr><td class="style2" style="width: 510px"><br /><asp:Label id="Label4" Text="Available dates" runat="server" Width="49%" Font-Bold="True"
ForeColor="Blue"></asp:Label></td><td style="height: 28px"><asp:DropDownList id="ddlbthdate" runat="server" Width="28%"
       AutoPostBack="True"></asp:DropDownList></td></tr><tr><td align = "center" class="style1" style="width: 510px"><br />&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; &#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; <asp:Button ID="Btnconfirmation" runat="server" OnClick="Btnconfirmation_Click" Text="Submit" Height="24px" Width="88px" /></td></tr></table></asp:Panel>

</ContentTemplate>
          <Triggers>
              <asp:PostBackTrigger ControlID="Btnentry"/>
          </Triggers>
     </asp:UpdatePanel>

When i run the above code shows error as follows
A control with ID 'Btnentry' could not be found for the trigger in UpdatePanel 'Updatepanel1'.

please help me.