Check if data already exists in Database on ListView Item Click using VB.Net in ASP.Net

How to display server-side error message when user click any item in listview without posting or submitting webform asp.net 

vb.net

Protected Sub ListView1_SelectedIndexChanging(sender As Object, e As ListViewSelectEventArgs)
 
     If dr.HasRows Then
 
         While dr.Read
 
         End While
     Else
        here I want display the error code
     End If
     '
 End Sub

<asp:ListView ID="ListView1" runat="server" DataKeyNames="GroupId"
   OnSelectedIndexChanging="ListView1_SelectedIndexChanging">
   <LayoutTemplate>
      <table  class="table" style="width: 75%" cellspacing="0" cellpadding="0" >
         <tr  style="background-color: #336699; color: White; border-bottom:solid; border-bottom-width:2px; border-bottom-color:black ">
         </tr>
         <tbody>
            <asp:PlaceHolder ID="itemPlaceHolder" runat="server" />
         </tbody>
      </table>
   </LayoutTemplate>  
   <ItemTemplate>
      <tr style="border-bottom:solid; border-bottom-width:2px">
         <td style="width: 60px">
            <asp:Button class="btn btn-default btn-lg btn-round" ID="lnkSelect" Text='<%# Eval("GroupId")%>' CommandName="Select" runat="server" Font-Size="X-Large"/>
         </td>
         <td style="font-family:'Trebuchet MS'; font-size:small; font-weight:bold"><%# Eval("GName")%> <br /> </td>
      </tr>
   </ItemTemplate>
   <SelectedItemTemplate>

before issuing token it will check if it has taken or not, that will be doing by serverside code vb.net