Error-The request was aborted: Could not create SSL/TLS secure channel

Hi

  I am getting below error but in Postman it is working fine

The request was aborted: Could not create SSL/TLS secure channel.

                var request = new RestRequest(Method.POST);
                request.AddHeader("Content-Type", "application/json");
                request.AddHeader("Token", "de4dc6");
                request.AddHeader("Gstin", "29AA1ZR");
                request.AddJsonBody(new
                {
                    supplyType = "O",
                    subSupplyType = "1",
                    subSupplyDesc = "",
                    docType = "INV",
                    docNo = "sum/2/24/179",
                    docDate = "07/07/2017",
                    fromGstin = "29AAFCC9980M1ZR",
                    fromTrdName = "welton",
                    fromAddr1 = "2ND CROSS NO 59  19  A",
                    fromAddr2 = "GROUND FLOOR OSBORNE ROAD",
                    fromPlace = "FRAZER TOWN",
                    fromPincode = 560090,
                    actFromStateCode = 29,
                    fromStateCode = 29,
                    toGstin = "29AEKPV7203E1Z9",
                    toTrdName = "sthuthya",
                    toAddr1 = "Shree Nilaya",
                    toAddr2 = "Dasarahosahalli",
                    toPlace = "Beml Nagar",
                    toPincode = 560090,
                    actToStateCode = 29,
                    toStateCode = 27,
                    transactionType = 4,
                    otherValue = "0",
                    totalValue = 56099,
                    cgstValue = 0,
                    sgstValue = 0,
                    igstValue = 300.67,
                    cessValue = 400.56,
                    cessNonAdvolValue = 400,
                    totInvValue = 68358,
                    transporterId = "",
                    transporterName = "",
                    transDocNo = "",
                    transMode = "1",
                    transDistance = "100",
                    transDocDate = "",
                    vehicleNo = "PVC1234",
                    vehicleType = "R",
                    itemList = new[]
                    {
                        new
                        {
                            productName = "Wheat",
                            productDesc = "Wheat",
                            hsnCode = 1001,
                            quantity = 4,
                            qtyUnit = "BOX",
                            cgstRate = 0,
                            sgstRate = 0,
                            igstRate = 3,
                            cessRate = 3,
                            cessNonadvol = 0,
                            taxableAmount = 5609889
                        }
                    }
                });
                IRestResponse response = client.Execute(request);

Thanks