The URL for exchanging a token should be https://id.twitch.tv/oauth2/token as mentioned in the docs https://dev.twitch.tv/docs/authentication/ The authorization URL where you initially redirect the user to is out of date too, so I suggest you update that.
The URL was changed many months ago and deprecated the old URL, so perhaps the issue is that they’ve simply disabled the old URL that has been out of date for many months.
Also, you’ve just disclosed your client secret so you should revoke that secret immediately.