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

I try to call a remote REST Web API. My tests in Postman are successful. but in .Net Framework 4.5.2 and using a HttpClient I get this error:

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

As mentioned Here, I set the followings:

System.Net.ServicePointManager.Expect100Continue = true;
System.Net.ServicePointManager.SecurityPortocol = System.Net.SecurityProtocolType.Tls12;

Also, I tried this one

but nothing has changed.

thanks for any help in advanced!