You must have [at least 50 reputation][1] in order to comment on other users' questions. See also the [Privileges][1] page.

> [...]

> Please note that you can always comment on your own posts, and any part of your questions. However, commenting on other people's posts is a privilege.

> ### How do I comment?

> Click the `add comment` link under each post. Comments cannot be multi-line, so simply press enter to submit your comment.

[1]: https://superuser.com/privileges/comment