Database Connect ion asp.net core API and ADO.NET

1. my json file

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },

  "ConnectionStrings": {
    "Database": "Data Source=;Initial Catalog=BookingService;TrustServerCertificate=True;User ID=sa; Password=PFCLP@08122022;Integrated Security=false;"
  },
  "AllowedHosts": "*"
}

2. Database Connection Method

  public static SqlConnection GetDbCon()
        {

            try
            {
                SqlConnection con = new SqlConnection();
                con.ConnectionString = System.Configuration.ConfigurationManager.ConnectionStrings["ConnectionStrings"].ConnectionString;
                return con;
            }

            catch (Exception ex)
            {
                throw ex;
            }

        }
 

3. Data Access Method

   public static List GetBookingDetailsPARA(string BookingRefNo)
        {
            using (SqlConnection SqlCn = DBMasterCon.GetDbCon())
            {
                SqlCommand cmd = new SqlCommand("SPVM_BookingReferenceTblPARA", SqlCn);
                cmd.CommandType = CommandType.StoredProcedure;

                cmd.Parameters.Add("@BookingRefNo", SqlDbType.VarChar, 50).Value = BookingRefNo;
                SqlCn.Open();

                List csd = new List();
                try
                {
                    using (SqlDataReader reader = cmd.ExecuteReader())
                    {
                        while (reader.Read())
                        {
                            BookingReferenceTbl grdcsd = new BookingReferenceTbl();

                            grdcsd.BookingRefNo = (string)Convert.ToString(reader["BookingRefNo"]);
                            grdcsd.ReturnOption = (string)System.Convert.ToString(reader["ReturnOption"]);
                            grdcsd.OneWayOption = (string)System.Convert.ToString(reader["OneWayOption"]);
                            grdcsd.MultiCityOption = (String)System.Convert.ToString(reader["MultiCityOption"]);
                            grdcsd.TravelFrom = (string)Convert.ToString(reader["TravelFrom"]);
                            grdcsd.TravelTo = (string)System.Convert.ToString(reader["TravelTo"]);
                            grdcsd.DepartTime = (DateTime)System.Convert.ToDateTime(reader["DepartTime"]);
                            grdcsd.ReturnDate = (DateTime)Convert.ToDateTime(reader["ReturnDate"]);
                            grdcsd.CabinClassTravelers = (string)System.Convert.ToString(reader["CabinClassTravelers"]);

                            csd.Add(grdcsd);
                        }

                        reader.Close();
                        return csd;
                    }
                }

                catch (Exception ex)
                {
                    throw ex;
                }

                finally
                {
                    SqlCn.Close();
                }
            }
        }

No i got the error on this implementation please help to resolve on this error