micah says:

cmd.CommandText = "INSERT INTO Sells VALUES('" + name + "','" + address + "','" + productName + "'," + unitPrice + "," + quantity + ")";

Check your insert statement there is problem. You need to specify the column name which you are going to insert. There is no any column name defined in insert statement you just return value clause where you are passing only values.

You have done calculation of multiplication of unitPrice and amount but not passed wit in insert statement with column name as Total. Refer the below link how to use insert stament with column name for your reference.

ASP.Net GridView- Insert Edit Update and Delete the ADO.NET way

i have done that see this answer here, working very well

                                //  int availableInventoryQty = AvailableInventoryQty();
                                if (lblProductPrice.Text.Length > 0 && txtQuantity.Text.Length > 0)
                                {

                                    double stock = Convert.ToDouble(txtQuantity.Text);
                                    double customerqty = Convert.ToDouble(txtQuantity.Text);
                                    double tqty = stock - customerqty;
                                    double gqty = tqty;
                                    {
                                        if (lblProductPrice.Text.Length > 0 && txtQuantity.Text.Length > 0)
                                        {

                                            double unitprice2 = Convert.ToDouble(lblProductPrice.Text);
                                            double customerqty2 = Convert.ToDouble(txtQuantity.Text);
                                            double grandtotal = unitprice2 * customerqty;
                                            double total = grandtotal;
                                            {

                                                {



                                                    if ((availableQuantity != 0) && (quantity != 0) && (quantity < availableQuantity))
                                                    {
                                                        using (SqlConnection con = new SqlConnection())
                                                        {
                                                            con.ConnectionString = str;
                                                            using (SqlCommand cmd = new SqlCommand())
                                                            {
                                                                cmd.CommandType = CommandType.Text;
                                                                cmd.Connection = con;
                                                                //Change the query like below
                                                                //Make sure the column name matches the with the name provided in code as per your table
                                                                cmd.CommandText = "INSERT INTO Sells (UserName,CustomerName,CustomerAddress,CustomerPhone,ProductCategory,UnitPrice,Quantity,PaymentStatus,TotalAmount) VALUES(@UserName,@CustomerName,@CustomerAddress,@CustomerPhone,@ProductCategory,@UnitPrice,@Quantity,@PaymentStatus,@TotalAmount)";
                                                                cmd.Parameters.AddWithValue("@UserName", Session["userName"]);
                                                                cmd.Parameters.AddWithValue("@CustomerName", ddlcustomername.SelectedItem.Text.Trim());
                                                                cmd.Parameters.AddWithValue("@CustomerAddress", ddlcustomeraddress.SelectedItem.Text.Trim());
                                                                cmd.Parameters.AddWithValue("@CustomerPhone", customerphone);
                                                                cmd.Parameters.AddWithValue("@ProductCategory", productcategory);
                                                                cmd.Parameters.AddWithValue("@UnitPrice", lblProductPrice.Text.Trim());
                                                                cmd.Parameters.AddWithValue("@Quantity", txtQuantity.Text.Trim());
                                                                cmd.Parameters.AddWithValue("@PaymentStatus", ddlpaymentstatus.SelectedItem.Text.Trim());
                                                                cmd.Parameters.AddWithValue("@TotalAmount", grandtotal);
                                                               // cmd.Parameters.AddWithValue("@OrderDate", DateTime.Today);
                                                                con.Open();
                                                                inserted = cmd.ExecuteNonQuery();
                                                                con.Close();
                                                            }
                                                        }
                                                    }
                                                    else
                                                    {
                                                        ClientScript.RegisterClientScriptBlock(this.GetType(), "", "alert('No of Quantity entered is not available in Stock')", true);
                                                    }
                                                    if (inserted > 0)
                                                    {
                                                        int updatedStock = availableQuantity - quantity;
                                                        using (SqlConnection con = new SqlConnection())
                                                        {
                                                            con.ConnectionString = str;
                                                            using (SqlCommand cmd = new SqlCommand())
                                                            {
                                                                cmd.CommandType = CommandType.Text;
                                                                cmd.Connection = con;
                                                                cmd.CommandText = "UPDATE Products SET Quantity = " + updatedStock + " WHERE ProductCategory = '" + productcategory + "'";
                                                                con.Open();
                                                                inserted = cmd.ExecuteNonQuery();
                                                                con.Close();
                                                            }
                                                        }
                                                    }
                                                    BindProductsSells();
                                                }