On the Twitch website, install the extension to a channel and test on Twitch instead.
It’s recommened to put your testing base URI being SSL, so you can accurately test.
Either using a webserver you uplaod files to, or ngrok or similar for a SSL enabled tunnel
You can read more about the deprecation here: End of support for the Twitch Developer Rig
Seems that you can’t add new projects using the UI since it tries to validate the details and those older API’s for that are dead/removed.