CallendarExtender is not a known element - asp.net

Question

I have two pages with the same code for ´UpdatePanel, ScriptManager and CalendarExtender, in one of them, theCalendarExtender` is working fine, but in the other, it's giving me this error:
CalendarExtender is not a known Element
Here's my code on aspx
<%# Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolKit" TagPrefix="asp" %>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder2" runat="server">
<div id="conteudo" style="text-align: left">
<fieldset id="fieldset" style="width:730px; margin-left: -200px">
<legend style="text-align:center;"><b>Detalhes do Chamado</b></legend>
<div id="DetalhesChamado">
<asp:ScriptManager ID="ScriptManager1" runat="server" EnableScriptGlobalization="true" EnableScriptLocalization="true" EnablePartialRendering="true"></asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
//Here is a gridview
<asp:CalendarExtender runat="server"></asp:CalendarExtender>
But still giving the error...
--Update 2--
Now i'm getting this error:
The TargetControlID of 'CalendarExtender1' is not valid. A control with ID 'TxtPrevisao' could not be found.
My New code:
<asp:TemplateField>
<ItemTemplate>
<asp:TextBox ID="TxtPrevisao" runat="server" Width="115px"></asp:TextBox>
<asp:CalendarExtender ID="CalendarExtender1" runat="server" Format=" dd ,MMMM ,yyyy" TargetControlID="TxtPrevisao" PopupButtonID="TxtPrevisao" CssClass="MyCalendar">
</asp:CalendarExtender>
</ItemTemplate>
</asp:TemplateField>

Answer 1

Change the Tag-Prefix of the Ajax-Kit an see if it's working
<%# Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="act" %>
And also:
<act:CalendarExtender runat="server"></act:CalendarExtender>
asp is used for Asp.Net components like the UpdatePanel
------------ Update 2 -------------------
To your new error, you can't assign the PopupButtonID to the textbox, you need a ImageButton
<asp:ImageButton ID="btnCalenderPopup" runat="server" ImageUrl="App_Themes/Standard/Img/calendar.png" CausesValidation="False" />
Still, I insist, you will need to assign the Tag-Prefix for the Took-Kit correctly and you will need to reference the assembly correctly as SanjeevRai said

Answer 2

You have to add reference of 'Ajaxtoolkit' dll into your project. It seems like it is not getting the proper reference of 'AjaxToolkit' DLL.
You can check references of your project by right clicking on website and select 'Property Pages' option. then it will show you all the references. from here you can check whether 'Ajaxtoolkit' reference exit in your project or not.
If Ajaxtoolkit reference does not exist in your project then you will have to add this reference first!
------------ Update 2 -------------------
After looking at your code I got the issue.
Actually you are using calenderextender and its TargetControlID textbox inside a gridview, so it is not getting the id of textbox.It will never identify the child control id directly on aspx page.
you need to attach 'TargetControlID' and 'PopupButtonID' at run time inside RowDataBound event of gridview.
Here is a sample of code that you can use in you aspx.cs file:
void myGridView_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow)
{
TextBox txt = (TextBox)e.Row.FindControl("TxtPrevisao");
CalendarExtender Calendar1 = (CalendarExtender)e.Row.FindControl("CalendarExtender1");
Calendar1.TargetControlID=txt.ID;
Calendar1.PopupButtonID=txt.ID;
}
}
And in .aspx page attach the rowdatabound event to grid and remove 'TargetControlID' and 'PopupButtonID' attributes from calenderextender i.e.
<asp:TemplateField>
<ItemTemplate>
<asp:TextBox ID="TxtPrevisao" runat="server" Width="115px"></asp:TextBox>
<asp:CalendarExtender ID="CalendarExtender1" runat="server" Format=" dd ,MMMM ,yyyy" CssClass="MyCalendar">
</asp:CalendarExtender>
</ItemTemplate>
</asp:TemplateField>