Hi,
First you need check closing tags for every control.
Remove "PopupButtonID" of Calendar Control(If you want calendar with text box click no use with image button).
Place the below code after page directive.
<add tagprefix="ajaxToolkit" namespace="AjaxControlToolkit" assembly="AjaxControlToolkit" />
Place the below code in body.
Choose Date:
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:TextBox ID="date1" runat="server"></asp:TextBox>
<asp:ImageButton ID="Image1" runat="server" />
<asp:CalendarExtender ID="CalendarExtender1" runat="server"
TargetControlID="date1"/>
I hope it will help you.