Ошибка подключения к ms sql server через c#

Имеется форма авторизации при вводе значения и нажатия на кнопку вход, появляется ошибка. P.S. Программный код описан ниже

using System; 
using System.Collections.Generic; 
using System.ComponentModel; 
using System.Configuration; 
using System.Data; 
using System.Data.SqlClient; 
using System.Drawing; 
using System.Linq; 
using System.Text; 
using System.Threading.Tasks; 
using System.Windows.Forms;

namespace Voyage
{
public partial class fAuthorization : Form
{
   string connectionString = ConfigurationManager.ConnectionStrings["SqlCon"].ConnectionString;
    SqlConnection connection;
    public fAuthorization()
    {
        InitializeComponent();
        topPanel.BackColor = Color.FromArgb(0, 71, 160);
        headerLabel.ForeColor = Color.White;
        lLogAndPassword.ForeColor = Color.FromArgb(0, 71, 160);
        signInBtn.BackColor = Color.FromArgb(0, 71, 160);
        lLog.ForeColor= Color.FromArgb(0, 71, 160);
        lPassword.ForeColor = Color.FromArgb(0, 71, 160);
        seePassword.ForeColor= Color.FromArgb(0, 71, 160);
        lCopyright.ForeColor = Color.FromArgb(0, 71, 160);
        shutdownBtn.BackColor= Color.FromArgb(0, 71, 160);
    }
    private void seePassword_CheckedChanged(object sender, EventArgs e)
    {
        if (seePassword.Checked) tbPassword.UseSystemPasswordChar = false;
        else tbPassword.UseSystemPasswordChar = true;
    }
    private void shutdownBtn_Click(object sender, EventArgs e)
    {
        this.Dispose();
    }
    private void signInBtn_Click(object sender, EventArgs e)
    {
        SqlDataReader reader;
        string role = "";
        string username = "";
        try
        {
            using(connection = new SqlConnection(connectionString))
            {
                string command = String.Format("Select sLog, sRole From tUser Where " +
                    "(sLog='{0}' Collate SQL_Latin1_General_CP1251_CS_AS) and " +
                    "(sPassword='{1}' Collate SQL_Latin1_General_CP1251_CS_AS)",
                    tbLog.Text, tbPassword.Text);
                SqlCommand Select = new SqlCommand(command, connection);
                reader = Select.ExecuteReader();
                if (reader.HasRows)
                {
                    while (reader.Read())
                    {
                        username = reader[0].ToString().Trim();
                        role = reader[1].ToString();
                    }
                    tbLog.Text = "";
                    tbPassword.Text = "";
                    MessageBox.Show(username, role);
                   /* MainMenu mm = new MainMenu(role, username);
                    mm.Owner = this;
                    mm.Show();
                    this.Hide();*/
                }
                else
                {
                    MessageBox.Show("Неверные логин или пароль",
                        "Ошибка",
                        MessageBoxButtons.OK,
                        MessageBoxIcon.Error);
                }
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.ToString());
        }
    }
}
}