I ran into the same error message while using the ChargifyNET.dll to communicate with the Chargify API. Adding chargify.ProtocolType = SecurityProtocolType.Tls12;
to the configuration solved the problem for me.
Here is the complete code snippet:
public ChargifyConnect GetChargifyConnect()
{
var chargify = new ChargifyConnect();
chargify.apiKey = ConfigurationManager.AppSettings["Chargify.apiKey"];
chargify.Password = ConfigurationManager.AppSettings["Chargify.apiPassword"];
chargify.URL = ConfigurationManager.AppSettings["Chargify.url"];
// Without this an error will be thrown.
chargify.ProtocolType = SecurityProtocolType.Tls12;
return chargify;
}