Make sure you've unpacked all plugin files and folders (twitch_db_client.py, twitch_launcher_client.py, twitch_plugin.py, aiohttp\, async_timeout\ e.t.c.) directly into appropriate folder: _c:\Users\\AppData\Local\GOG.com\Galaxy\plugins\installed\twitch8b831aed-dd5f-c0c5-c843-41f9751f67a2.
If this still doesn't work for you, please contact GOG support.
Downloaded the latest version and put it in the plugin folder, restarted GLX, but it's not available to connect to.