A ScriptManager is required on the page to use ASP.NET AJAX Script Components

<%--添加AjaxControlToolkit引用--%>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="ajaxToolkit" %>

 

 

添加 这个就行了

<form id="Form1" method="post" runat="server">
   <asp:ScriptManager ID="ScriptManager1" runat="server">
   </asp:ScriptManager>