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

From @sameerfair comment

"For .Net v4.0 I noticed, setting the value of ServicePointManager.SecurityProtocol to (SecurityProtocolType)3072 but before creating the HttpWebRequest object helped."

The above suggestion worked for me. Below are my code lines which are worked for me

var securedwebserviceurl="https://somedomain.com/service";
ServicePointManager.Expect100Continue = true;
ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3 | SecurityProtocolType.Tls12 | SecurityProtocolType.Tls | SecurityProtocolType.Tls11;
// Skip validation of SSL/TLS certificate
ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };
var httpWebRequest = (HttpWebRequest)WebRequest.Create(securedwebserviceurl);
httpWebRequest.ContentType = "application/json";
httpWebRequest.Method = "POST";
httpWebRequest.ProtocolVersion= HttpVersion.Version10;
var httpResponse = (HttpWebResponse)httpWebRequest.GetResponse();
using (var streamReader = new StreamReader(httpResponse.GetResponseStream())) {
string responseFromServer = streamReader.ReadToEnd();
}

This worked for me :

ServicePointManager.Expect100Continue = true;
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls
| SecurityProtocolType.Tls11
| SecurityProtocolType.Tls12
| SecurityProtocolType.Ssl3;

This worked for me :

ServicePointManager.ServerCertificateValidationCallback = (snder, cert, chain, error) => true;

if it doesn't work, it may work with other solutions like this :

ServicePointManager.SecurityProtocol = SecurityProtocolType.Ssl3;
System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;
ServicePointManager.ServerCertificateValidationCallback = (snder, cert, chain, error) => true;

Unfortunately none of the answers mentioned above worked for me.
Below listed code did a wonder for me. In case if it helps someone.

ServicePointManager.Expect100Continue = true;
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls
| SecurityProtocolType.Tls11
| SecurityProtocolType.Tls12
| SecurityProtocolType.Ssl3;

And this must set before creating the HttpWebRequest.

It can be useful when the server where your application is hosted supports only Tls 1.2. In this case you'll need remove old insecure protocols, and keep only Tls 1.2:

```c#
// Remove insecure protocols (SSL3, TLS 1.0, TLS 1.1)
ServicePointManager.SecurityProtocol &= ~SecurityProtocolType.Ssl3;
ServicePointManager.SecurityProtocol &= ~SecurityProtocolType.Tls;
ServicePointManager.SecurityProtocol &= ~SecurityProtocolType.Tls11;

// Add TLS 1.2
ServicePointManager.SecurityProtocol |= SecurityProtocolType.Tls12;

```

This error showed up for me as a **symptom of AppLocker blocking a PowerShell script**. As soon as I ran it from a whitelisted location, no more issues.