In this post already a user with 1 reputation(at the time of asking this question) has commented but when I tried to add comment, its showing "You must have 50 reputation to comment". Why am I not able to comment with 13 reputations when user with 1 reputation can comment?
The comment you see was actually an answer post converted to a comment by a moderator. ♦Moderators can convert answers to comments provided they are no more than 600 characters long. (Perks of being a 10k user—I can see deleted answers most users cannot.)
This typically happens when a user intends to post a comment and cannot do so for the very reason you described: insufficient reputation. The restriction simply exists to protect against spam. Users with less than 50 reputation can still comment on their own questions and their answers.
I'm the moderator that converted that, and your mileage, as far as comment conversions may vary.
In general we prefer that users not post answers for conversion, but rather get the necessary reputation to do so. We may, however see a comment that is useful, and rather than outright deletion, choose to convert it. In general, if you know what a comment is/should be, you'd have the rep to post those anyway.
In this case, well, the suggestion was extremely useful, and I felt it didn't deserve to be canned. The user didn't flag it for conversion.