i try my best to identify the problem and make some changes in my code. can anyone explain where and what is my wrong in my code but still have this error "A page can have only one server-side Form tag".

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




public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }

    public string GetConnectionString()
    {
        return System.Configuration.ConfigurationManager.ConnectionStrings["ASPNETDBConnectionString"].ConnectionString;
    }

    private void ExecuteInsert(string customer_id, string name, string gender, string calculate, string address, string nationality, string email, string mobile, string license, string rentdate, string returndate, string total)
    {


        SqlConnection conn = new SqlConnection (GetConnectionString());

        string sql = "INSERT INTO rental (customer_id, name, gender, address, nationality, email, mobile, license, rentdate, returndate, calculate, total) VALUES " + " (customer_id, name, gender, address, nationality, email, mobile, license, rentdate, returndate, calculate, total)";

        try
        {
            conn.Open();

            SqlCommand cmd = new SqlCommand(sql, conn);
            SqlParameter[] param = new SqlParameter[12];


            param[0] = new SqlParameter("customer_id",SqlDbType.VarChar, 50);
            param[1] = new SqlParameter("name", SqlDbType.VarChar, 50);
            param[2] = new SqlParameter("gender", SqlDbType.VarChar, 50);
            param[3] = new SqlParameter("address", SqlDbType.VarChar, 50);
            param[4] = new SqlParameter("nationality", SqlDbType.VarChar, 50);
            param[5] = new SqlParameter("email", SqlDbType.VarChar, 50);
            param[6] = new SqlParameter("mobile", SqlDbType.VarChar, 50);
            param[7] = new SqlParameter("licence", SqlDbType.VarChar, 50);
            param[8] = new SqlParameter("rentdate", SqlDbType.VarChar, 50);
            param[9] = new SqlParameter("returndate", SqlDbType.VarChar, 50);
            param[10] = new SqlParameter("calculate", SqlDbType.VarChar, 50);
            param[11] = new SqlParameter("total", SqlDbType.VarChar, 50);



            param[0].Value = customer_id;
            param[1].Value = name;
            param[2].Value = gender;
            param[3].Value = address;
            param[4].Value = nationality;
            param[5].Value = email;
            param[6].Value = mobile;
            param[7].Value = license;
            param[8].Value = rentdate;
            param[9].Value = returndate;
            param[10].Value = calculate;
            param[11].Value = total;

            

            for (int i = 0; i < param.Length; i++)
            {
                cmd.Parameters.Add(param[i]);
            }
            cmd.CommandType = CommandType.Text;
            cmd.ExecuteNonQuery();
        }
        catch (System.Data.SqlClient.SqlException ex)
        {
            string msg = "Insert error:";
            msg += ex.Message;
            throw new Exception(msg);
        }
        finally
        {
            conn.Close();
        }
    }
    protected void btnsave_Click(object sender, EventArgs e)
    {
        ExecuteInsert(txtcustomer_id.Text, txtname.Text, txtgender.Text, txtaddress.Text, txtnationality.Text, txtemail.Text, txtmobile.Text, txtlicence.Text, txtrentdate.Text, txtreturndate.Text, txtcalculate.Text, txttotal.Text);

        Response.Write("Record was succesfully added!");
        
    }
    protected void txtcalculate_TextChanged(object sender, EventArgs e)
    {

    }
}

hi. thank you for your post. but can you explain more deeply witch can make me understand. thank you