The problem is with this tag <button id="Trigger">Trigger Slideup/SlideDown</button>
in Asp.net we can use
<asp:Button ID="Button1" runat="server" Text="Button"/>
for server control or
<input type="button" id="Button1" value="Search" />
for html control.
so try to replace <button id="Trigger">Trigger Slideup/SlideDown</button> to
<asp:Button ID="Button1" runat="server" Text="Button"/>
or to this
<input type="button" id="Button1" value="Search" />
based on your requirement.