There is already an open DataReader associated with this Command which must be closed first.

using System.Data;
using System.Data.SqlClient;
using System.Configuration;
public partial class Finaldestination : System.Web.UI.Page
{
    SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["dbconnection"].ConnectionString);
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        string strSelect = "SELECT * FROM tblregistration  WHERE UserId = '" + TextBox2.Text + "' ";
        SqlCommand cmdSelect = new SqlCommand(strSelect,con);
        cmdSelect.Parameters.AddWithValue("@UserId", TextBox2.Text);

        cmdSelect.CommandText = strSelect;
        cmdSelect.Connection = con;
        SqlDataReader reader;
        con.Open();
        reader = cmdSelect.ExecuteReader();
        {
            
            if ( reader.HasRows == true)
            {
                con.Close();
                Label1.Text = "Username exists please try other one";
            }
            
            else
            {

                string insert = "insert into tblregistration(Name,UserId,Email,Password,Confirmpassword) values (@Name,@UserId,@Email,@Password,@Confirmpassword) ";
                SqlCommand cmd = new SqlCommand(insert, con);
                cmd.Parameters.AddWithValue("@Name", TextBox1.Text);
                cmd.Parameters.AddWithValue("@UserId", TextBox2.Text);
                cmd.Parameters.AddWithValue("@Email", TextBox3.Text);
                cmd.Parameters.AddWithValue("@Password", TextBox4.Text);
                cmd.Parameters.AddWithValue("@Confirmpassword", TextBox5.Text);
                //con.Open();
                cmd.ExecuteNonQuery();  
                Label1.Text = "data saved";
               
            }
        }