Welcome to Math.SE. Your post and Comments are somewhat ambiguous about what you wanted to accomplish, e.g. post an Answer to the linked Question or merely to Comment on what is now a five and a half year old Question.
Different amounts of reputation are required for these actions. Since the Question is "protected", one needs to have earned $10$ points for the privilege of posting an Answer there. This mechanism of protected Questions is meant to limit the number of low-quality Answers that must go through a review process.
On the other hand you perhaps wanted to have the observation posted as a Comment. It is true that Comments in the StackExchange model are considered ephemeral content, so that completeness is not expected. However $50$ points are required for the privilege of commenting on the posts of others (you can always comment on your own Questions and Answers, and on the posts of others that answer your Questions).
In commenting above on feedback about the privilege requirements, you wrote, "I don't want to build reputation." Note that reputation here is built primarily by contributing good content to the Community. Perhaps you will accomplish this without special effort, knowing that the rules are intended to collect excellent content for students of mathematics at all levels.