Paypal沙箱错误：请求已中止：无法创建SSL / TLS安全通道

我收到错误消息“请求已中止：无法创建SSL / TLS安全通道。” 如果是沙箱，则可以使用我的实时凭据。

这是代码片段

            var uri = new Uri(url);
            ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
            var request = WebRequest.Create(uri);
            var encoding = new UTF8Encoding();
            var requestData = encoding.GetBytes(postData);

            request.ContentType = "application/x-www-form-urlencoded";
            request.Method = "POST";
            request.Timeout = (300 * 1000); //TODO: Move timeout to config
            request.ContentLength = requestData.Length;

            using (var stream = request.GetRequestStream())
            {
                stream.Write(requestData, 0, requestData.Length);
            }

            var response = request.GetResponse();

            string result;

            using (var reader = new StreamReader(response.GetResponseStream(), Encoding.ASCII))
            {
                result = reader.ReadToEnd();
            }

            return result;