<p>New users cannot comment until they reach <a href="https://stats.stackexchange.com/help/privileges/comment">50 reputation</a>, not 15.  This is part of the design of the SE system.  It is a feature, not a bug.  Preventing new users from being able to comment reduces the amount of spam and nonsense on the site and help keep it clean.  On balance, the benefits of this design choice far out weigh the costs.  Sooner or later, someone with the privilege will comment on the thread.</p>
<p>You should flag the (non) answer for deletion.  It is nice to leave a polite comment for the OP explaining that they need to earn the reputation before they can comment, and that they should not post comments as pretend answers in the interim.  As @Glorfindel notes, moderators can convert the answer to a comment, if they think it best.  I used to do a lot of that when I was a moderator.</p>
