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.
Then is it OK to answer for a post to which we want to comment and ask the moderator in that same answer to convert it to comment?– learnerNov 4, 2014 at 5:32
5You really shouldn't do that. Instead, you should post some good-quality answers to other questions and earn the required reputation. I know this can be frustrating, but we wouldn't have such a high-quality, spam-free Q&A experience without these stringent restrictions.– bwDracoNov 4, 2014 at 5:35
4I also see you have 192 reputation on Ask Ubuntu. You just need two question upvotes or one answer upvote there to get the +100 association bonus on all other sites, including Super User, and this would resolve your problem.– bwDracoNov 4, 2014 at 5:49
3@learner - No; You shouldn't create more work for the moderators and everyone in the community by posting an answer that is only a comment. Get the reputation then post the comment.– RamhoundNov 4, 2014 at 16:19
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.