This would suggest the the cert bundle, that you have installed with Unity is out of date. And a Unity update or sub module update needs to be installed
A brief google suggests:
- Unity SSL TlsException - Questions & Answers - Unity Discussions
- HttpWebRequest.GetRequestStream() https certificate error exception - Questions & Answers - Unity Discussions
- https://forum.unity.com/threads/ssl-certificate-validation.222529/
This looks like a issue with Unity here rather than Twitch itself?