What @notjoker28 said is correct, not all channels use Nightbot, but in case you think you don’t see it when you should, type /unignore Nightbot (not /unblock) in any chat, and that will stop hiding Nightbot’s messages if it was (also specified in the troubleshooting guide).
It wasn’t clear you were having issues as a viewer, this would have made my first response very different, that’s why we ask people to be specific. We assume that you’re having issues setting up Nightbot as a streamer, as it’s the most common occurrence when people say they “can’t see Nightbot in chat.” Therefore, a lot of our advice was out of topic for your issue because of this, way too advanced, you can disregard most of it. Describing your issue accurately saves time and prevents hassle.