facing problem with this error in query

using System;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;
using System.Web.Configuration;
using System.Drawing;

public partial class InventoryHome : System.Web.UI.Page
{
    private static readonly string _connString = String.Empty;
    SqlConnection con = new SqlConnection(_connString);

    protected void Page_Load(object sender, EventArgs e)
    {
         if (!IsPostBack)
        {
            LoadInventoryData();

        }
    }


    // ADD RECORD USING FOOTER
    protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
    {
        if (e.CommandName.Equals("AddNew"))
        {
            DropDownList ddlEquipmenttype = (DropDownList)GridView1.FooterRow.FindControl("ddlftrEquipment_type");
            DropDownList ddlMaker = (DropDownList)GridView1.FooterRow.FindControl("ddlftrMaker");
            TextBox txtModel = (TextBox)GridView1.FooterRow.FindControl("txtftrModel");
            TextBox txtSerialno = (TextBox)GridView1.FooterRow.FindControl("txtftrSerialno");
            TextBox txtKKIATagNo = (TextBox)GridView1.FooterRow.FindControl("txtftrKKIATagNo");
            TextBox txtGACATagNo = (TextBox)GridView1.FooterRow.FindControl("GACATagNo");
            TextBox txtAssignedto = (TextBox)GridView1.FooterRow.FindControl("txtftrAssignedto");
            TextBox txtLocation = (TextBox)GridView1.FooterRow.FindControl("txtftrLocation");


            con.Open();


            SqlCommand cmd = new SqlCommand("Insert into Equipment_Inventory (Equipment_type,Maker,Model,Serialno,KKIATagNo,GACATagNo,Assignedto,Location) Values ('" + ddlEquipmenttype.SelectedItem.Text + "','" + ddlMaker.SelectedItem.Text + "','" + txtModel.Text + "', '" + txtSerialno.Text + "','" + txtKKIATagNo.Text + "','" + txtGACATagNo.Text + "','" + txtAssignedto.Text + "','" + txtLocation.Text + "')", con);
            int result = cmd.ExecuteNonQuery();
            con.Close();
            if (result == 1)
            {
                LoadInventoryData();
                /////lblresult.ForeColor = Color.Green;//////////
                /////lblresult.Text = txtUsrname.Text + " Details inserted successfully";/////////
                ScriptManager.RegisterStartupScript(this, this.GetType(), "RunCode", "javascript:alert('Inventory added Successfully');", true);
            }
            else
            {
                /////lblresult.ForeColor = Color.Red;///////////
                // lblresult.Text = txtFlightNo.Text + " Details not inserted";////////////
            }


        }

    }


    // LOAD INVENTORY DATA
    private void LoadInventoryData()
    {
        SqlConnection con = new SqlConnection(_connString);


        SqlCommand cmd = new SqlCommand("Select * from Equipment_Inventory", con);
        //SqlCommand cmd = new SqlCommand("Select Dept_code as DeptCode, Dept_desc as Description, Dept_telno as TelephoneNo, Dept_dirmgr as DirectorManager, Equipment_type as EquipmentType, Maker as Maker, Model as Model, Serialno as SerialNo, TagNo as TagNo, Assignedto as AssignedTo, Location as Location from Equipment_Inventory", con);
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        DataSet ds = new DataSet();
        da.Fill(ds);
        GridView1.DataSource = ds;
        GridView1.DataBind();


    }


    // SUBMIT INVENTORY
    protected void BtnSubmitInventory_Click(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(_connString);
        con.Open();
        SqlDataAdapter da = new SqlDataAdapter("Select * from Equipment_Inventory", con);
        SqlCommandBuilder builder = new SqlCommandBuilder(da);
        DataSet ds = new DataSet("Equipment_Inventory");
        da.Fill(ds, "Equipment_Inventory");

        DataTable Equipment_InventoryTable = ds.Tables["Equipment_Inventory"];
        DataRow row = Equipment_InventoryTable.NewRow();

        row["Dept_code"] = TxtDepartmentCode.Text;
        row["Dept_desc"] = TxtDescription.Text;
        row["Dept_telno"] = TxtTelephoneNo.Text;
        row["Dept_dirmgr"] = TxtDirectorManager.Text;

        row["Dentry"] = DateTime.Now.ToString();


       
        Equipment_InventoryTable.Rows.Add(row);
        da.Update(ds, "Equipment_Inventory");

        
        con.Close();

       
        TxtDepartmentCode.Text = "";
        TxtDescription.Text = "";
        TxtTelephoneNo.Text = "";
        TxtDirectorManager.Text = "";

        ScriptManager.RegisterStartupScript(this, this.GetType(), "RunCode", "javascript:alert('Inventory added Successfully');", true);

    }


    static InventoryHome()
    {
        _connString = WebConfigurationManager.ConnectionStrings["Inventory_DBConnectionString"].ConnectionString;
    }

   
}

SqlCommand cmd = new SqlCommand("Insert into Equipment_Inventory (Equipment_type,Maker,Model,Serialno,KKIATagNo,GACATagNo,Assignedto,Location) Values ('" + ddlEquipmenttype.SelectedItem.Text + "','" + ddlMaker.SelectedItem.Text + "','" + txtModel.Text + "', '" + txtSerialno.Text + "','" + txtKKIATagNo.Text + "','" + txtGACATagNo.Text + "','" + txtAssignedto.Text + "','" + txtLocation.Text + "')", con);