WCF + PostgresQL. Npqsql connection is not open

Question

I'm creating a client-server chat application using WCF. To connect I use the same code as for my other app, but it doesn't work here. This is part of my Code

Client:

private void bReg_Click(object sender, RoutedEventArgs e)
{
    client = new ServiceChatClient(new System.ServiceModel.InstanceContext(this));
    string response = client.Registration(tbUserName.Text,tbPassword.Text);
    MessageBox.Show(response);
    string[] responseSub = response.Split('#');
    if (responseSub[1] == string.Empty)
    {
        MessageBox.Show(responseSub[0]);
    }
    else
    {
        MessageBox.Show(responseSub[0]);
    }
}

ServiceChat:

 string connstring = String.Format("Server={0};Port={1};" + "User ID = {2};Password={3};Database={4};", "localhost", "5432", "postgres", "mypass", "users");
 public NpgsqlCommand cmd;
 public string sql = null;

    public string Registration(string username, string password)
    {
        try 
        {
            NpgsqlConnection conn = new NpgsqlConnection(connstring);
            conn.Open();
            sql = @"SELECT * FROM users where username = :username";
            NpgsqlCommand cmd = new NpgsqlCommand(sql, conn);
            cmd.Parameters.AddWithValue(":username", username);
            int result = Convert.ToInt32(cmd.ExecuteScalar());
            if (result == 0)
            {
                sql = ("INSERT INTO USERS (username,password) VALUES (:username,:password)");
                cmd = new NpgsqlCommand(sql, conn);
                if (password.Length >= 4 && username.Length > 0 && password.Length <= 128 &&     
                    username.Length <= 128 && !password.Contains(' '))
                {
                    cmd.Parameters.AddWithValue("username", username);
                    cmd.Parameters.AddWithValue("password", GetMD5(password));
                    cmd.ExecuteNonQuery();
                    conn.Close();
                    return "Registration completed successfully#OK";
                }
                else
                {
                    conn.Close();
                    return "Invalid username or password#";
                }
            }
            else
            {
                conn.Close();
                return "A user with the same username already exists#";
            }
        } 
        catch (Exception ex)
        {
            string error = string.Format("Error:{0} что-то пошло не так#", ex); 
            return error;
        }

    }

I tried clearing the NpgsqlConnection pool but it didn't help.I just started working with WCF, so there may be a mistake somewhere else