ช่วยดู error ให้ทีค่ะ ดึงข้อมูลจาก database ขึ้นมา +1 แล้วเก็บเข้าไปใหม่

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data; 
using System.Data.SqlClient;
using System.Globalization;

namespace IT_REQ
{
    public partial class _Default : System.Web.UI.Page
    {
        

        protected void Page_Load(object sender, EventArgs e)
        {
            
        }
        

        protected void submit_Click(object sender, EventArgs e)
        {
            // ส่วน connect //
            SqlConnection objConn;
            String strConnString;
            strConnString = "Server=*****;Uid=**;PASSWORD=***;database=test;";
            objConn = new SqlConnection(strConnString);
            objConn.Open();

            //************** ทดสอบ connect to database ***********//
            if (objConn.State == ConnectionState.Open)
            {
                this.lblText.Text = "SQL Server Connected";
            }
            else
            {
                this.lblText.Text = "SQL Server Connect Failed";
            }

            SqlTransaction tr = objConn.BeginTransaction();
            SqlCommand cmd = new SqlCommand();
            cmd = objConn.CreateCommand();
            cmd.CommandType = CommandType.Text;
            cmd.Transaction = tr;
            
            // date //
            DateTimeFormatInfo dateFormat = new CultureInfo("en-US").DateTimeFormat;
            string d = DateTime.Now.ToString("MM/dd/yyyy", dateFormat);
            // ส่วน query //
            //////////////////// *******  zone error *********//////////////////////
            string n = "SELECT no+1 FROM REQUEST ORDER BY no DESC";
            //cmd.CommandText = n;
            string n2 = n;
            Convert.ToInt16(n2);
            ///////////////////////////////////////////////////////////////////////
            string isUS = "INSERT INTO REQUEST(no,plant,name,dep,tel,date,type) " + 
                "VALUES('"+n2+"','"//ID
                          +this.DropDownList4.Text+"','"//name
                          +this.TextBox1.Text+"','"//plant
                          +this.TextBox2.Text+"','"//department
                          +this.TextBox3.Text+"','"//tel
                          +d+"','"//date
                          +this.CheckBox1.Text+"')";//type
            
            cmd.CommandText = isUS;
            cmd.ExecuteNonQuery();
            tr.Commit();


            
            objConn.Close();
            objConn = null;
            tr.Dispose();

            // clear //
            TextBox1.Text = ""; //name
            DropDownList4.ClearSelection();//plant
            TextBox2.Text = "";//department
            TextBox3.Text = "";//tel
            CheckBox1.Checked = false;//type
        }

    }
}