I've been thinking lately of reacting to series and seeing how it works. But I do not know what the legal situation looks like. Is it allowed to send something to live or are there rules from Twitch (etc.) or laws against it?
You can certainly stream your reactions live, also on Twitch, YouTube and co, but of course you can't show the series in question, they would have to watch your viewers in TV / Netflix / Whatever.
That's exactly how it looks. The series has a copyright, which would be violated if you just broadcast it in a reaction video.