I signed up to contribute to a conversation about a movie but this merit system is absolutely ridiculous and I'd like to immediately disconnect myself from this website because I get nothing from it if I have to establish a reputation just to comment.
Please note that "reputation" doesn't mean you have to spend ages here, get known to the regular users, or anything like that - you don't need a "reputation" in the ordinary sense of the word. The only thing you need to do to get Stack Exchange reputation is to post questions or answers which get upvoted. Just 5 upvotes on a post of yours is enough to give you 51 reputation, which is enough to leave comments everywhere. There's no reputation requirement for posting answers (although they must actually answer the question asked), nor for posting comments on your own posts.
It may seem like needless jumping through hoops, but the purpose is to avoid having the site flooded with random commentary by anyone who feels like expressing an opinion. If you have an answer to a question, that's useful content for the Q&A site and you can always provide that even as a brand-new user. But if anyone could post comments, it would be much harder to filter out the really useful information.
I mention this in case you misunderstood what's meant by "you need reputation to comment".