The TargetControlID of 'modelPopupExtender1' is not valid. A control with ID 'btn' could not be found.

<form id="form1"  runat="server">
    <div>
 <asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server"/>

            <asp:UpdatePanel ID="UpdatePanel1" runat="server">
           
             <contenttemplate>
             <input type="button" id="btn" value="Okay" />
  <asp:ModalPopupExtender runat="server" 
                        ID="modelPopupExtender1" 
                        TargetControlID="btn"
                        PopupControlID="popUpPanel" 
                        
                        BackgroundCssClass="modalBackground">
 

<asp:Panel ID="popUpPanel" runat="server" CssClass="pnl" Visible="false">
 <div style="font-weight: bold; border: Solid 3px Aqua; <br mode=" hold=" />                                background-color: AliceBlue">
 
 <asp:Label runat="server" ID="myThrobber" Style="display: none;"><img align="absmiddle" alt="" src="images/uploading.gif"/>
                <asp:AjaxFileUpload ID="AjaxFileUpload1" 
                    runat="server" 
                    OnUploadComplete="AjaxFileUpload1_UploadComplete"
                    OnClientUploadError="uploadError" 
                    OnClientUploadComplete="uploadComplete"
                    ThrobberID="myThrobber"
                    MaximumNumberOfFiles="1"
                    AllowedFileTypes="" 
                    class="name_text1" 
                    Width="200px" />
                <asp:Label ID="Label1" runat="server" Text="">
                <asp:Button ID="BtnClose" runat="server" Text="Close" 
                    OnClientClick="Closepopup()"/>
                
                <asp:Label ID="Label2" runat="server" Text="">
 
                <br />
    
    </form>