始终获得异常请求已中止：无法创建SSL / TLS安全通道

始终获得异常请求已中止：无法创建SSL / TLS安全通道。

请帮助

Always getting exception The request was aborted: Could not create SSL/TLS secure channel.
Please help

CallerServices caller = new CallerServices();

            IAPIProfile profile = ProfileFactory.createSignatureAPIProfile();

            // Set up your API credentials, PayPal end point, and API version.
            profile.APIUsername = ConfigurationManager.AppSettings["APIusername"].ToString();
            profile.APIPassword = ConfigurationManager.AppSettings["APIpassword"].ToString();
            profile.APISignature = ConfigurationManager.AppSettings["APIsignature"].ToString();
            profile.Environment = ConfigurationManager.AppSettings["APIEnv"].ToString();
            caller.APIProfile = profile;

            // Create the request object.
            SetExpressCheckoutRequestType pp_request = new SetExpressCheckoutRequestType();
            pp_request.Version = "51.0";

            BillingAgreementDetailsType billingAgreement = new BillingAgreementDetailsType();
            billingAgreement.BillingType = BillingCodeType.MerchantInitiatedBilling;// BillingCodeType.RecurringPayments;
            billingAgreement.BillingAgreementDescription = "Membership Payment";
            List<BillingAgreementDetailsType> agreementList = new List<BillingAgreementDetailsType>();
            agreementList.Add(billingAgreement);

           
            pp_request.SetExpressCheckoutRequestDetails = new SetExpressCheckoutRequestDetailsType();
            pp_request.SetExpressCheckoutRequestDetails.PaymentAction = PaymentActionCodeType.Sale;//Enum for PaymentAction is  PaymentActionCodeType.Sale
            pp_request.SetExpressCheckoutRequestDetails.PaymentActionSpecified = true;
            pp_request.SetExpressCheckoutRequestDetails.BillingAgreementDetails = agreementList.ToArray();
            

            pp_request.SetExpressCheckoutRequestDetails.OrderTotal = new BasicAmountType();

            pp_request.SetExpressCheckoutRequestDetails.OrderTotal.currencyID = CurrencyCodeType.USD;//Enum for currency code is  CurrencyCodeType.USD

            pp_request.SetExpressCheckoutRequestDetails.OrderTotal.Value = paymentAmount;
            pp_request.SetExpressCheckoutRequestDetails.OrderDescription = "Membership Subscription";

            pp_request.SetExpressCheckoutRequestDetails.NoShipping = "1";
            pp_request.SetExpressCheckoutRequestDetails.BuyerEmail = buyerPaypalId;

            pp_request.SetExpressCheckoutRequestDetails.CancelURL = ConfigurationManager.AppSettings["HTTPSInitial"].ToString() + cancelurl;
            pp_request.SetExpressCheckoutRequestDetails.ReturnURL = ConfigurationManager.AppSettings["HTTPSInitial"].ToString() + returnurl;
            
            
            
            

            // Execute the API operation and obtain the response.
            SetExpressCheckoutResponseType pp_response = new SetExpressCheckoutResponseType();
            pp_response = (SetExpressCheckoutResponseType)caller.Call("SetExpressCheckout", pp_request);