Model popup is not showing on ASP grid view link feild

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C#

ASP.NET

HTML

Ajax

The below is the HTML code

<%@ Page Title="<%$Resources:MenuResource,SBSHomePage%>" Language="C#" MasterPageFile="~/MainMenu.Master" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="SBS.Default1" %>

<%@ Register Assembly="AjaxControlToolkit" 
             Namespace="AjaxControlToolkit" 
             TagPrefix="AjaxToolkit" %>



<asp:Content ID="Content1" ContentPlaceHolderID="HeadContent" runat="server">
  <style type="text/css">
.modalBackground 
{
 background-color:Gray;filter:alpha(opacity=70);opacity:0.7;
} 
</style>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
   

    <asp:HiddenField ID="hdnlang" runat="server" />
    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
   <AjaxToolkit:ToolkitScriptManager ID="scriptManager" 
                                      runat="server">
    </AjaxToolkit:ToolkitScriptManager>

                <asp:GridView ID="grdAPTransaction" runat="server" AllowPaging="True" AutoGenerateColumns="False"   CssClass="GridControl" >
<Columns>
<asp:TemplateField HeaderText="Quotation">
<ItemTemplate>
<asp:LinkButton runat="server" ID="lnkCustomer"  Text='<%#Eval("Quotation") %>' OnClick="lnkCustomer_Click">
</asp:LinkButton>
</ItemTemplate>
</asp:TemplateField>
    <asp:TemplateField HeaderText="PurchaseOrder">
<ItemTemplate>
<asp:LinkButton runat="server" ID="lnkCustomer1"  Text='<%#Eval("PurchaseOrder") %>' >
</asp:LinkButton>
</ItemTemplate>
</asp:TemplateField>
    <asp:TemplateField HeaderText="DeliveryOrder">
<ItemTemplate>
<asp:LinkButton runat="server" ID="lnkCustomer2"  Text='<%#Eval("DeliveryOrder") %>' >
</asp:LinkButton>
</ItemTemplate>
</asp:TemplateField>
    <asp:TemplateField HeaderText="Invoice">
<ItemTemplate>
<asp:LinkButton runat="server" ID="lnkCustomer3"  Text='<%#Eval("Invoice") %>' >
</asp:LinkButton>
</ItemTemplate>
</asp:TemplateField>
    <asp:TemplateField HeaderText="Payvoucher">
<ItemTemplate>
<asp:LinkButton runat="server" ID="lnkCustomer4"  Text='<%#Eval("Payvoucher") %>' >
</asp:LinkButton>
</ItemTemplate>
</asp:TemplateField>

</Columns>
</asp:GridView>
                <asp:Button runat="server" ID="btnModalPopUp" style="display:none"/>
 <AjaxToolkit:ModalPopupExtender ID="modalPopUpExtender1" runat="server"   TargetControlID="btnModalPopUp"  PopupControlID="pnlPopUp"   OkControlID="btnOk"   X="220"    Y="300">
</AjaxToolkit:ModalPopupExtender>
<asp:Panel runat="Server" ID="pnlPopUp" >
<asp:GridView ID="grdARTransaction" runat="server">
</asp:GridView>
</asp:Panel>
 </ContentTemplate>
</asp:UpdatePanel>
</asp:Content>

What I have tried:

and the CS backend code Code is

protected void Page_Load(object sender, EventArgs e)
       {
           if (Session["CLanguage"] != null)
               hdnlang.Value = Session["CLanguage"].ToString();

           DataTable dt = new DataTable();
           DataTable dt1 = new DataTable();
           BusinessLayer.SBS_CRM crm = new BusinessLayer.SBS_CRM();
           BusinessLayer.SBS_CRMs crms = new BusinessLayer.SBS_CRMs();

           dt = crm.selectAPTransaction();

           grdAPTransaction.DataSource = dt;
           grdAPTransaction.DataBind();

            }

       protected void lnkCustomer_Click(object sender, EventArgs e)
       {
          DataTable dt = new DataTable();
           dt = crm.selectAPTransaction();


           grdARTransaction.DataSource = dt;
           grdARTransaction.DataBind();

           modalPopUpExtender1.Show();
       }



   }

The grid is displaying with the link feild
when i click on the click the modelpopup is not showing