Flagging requires reputation because we do not want every newly registered user to be able to flag. As @rene says, it takes a while to know what needs to be flagged and what not (even 15 reputation is rather low). Lowering the reputation threshold will make reporting easier but lead to more noise, which makes flag handling (by the community in the review queues, and by ♦ moderators) a lot less efficient.
Note that an exception has been made (as part of the Welcoming initiative) for flagging comments on your own posts; for that, no reputation is required. If you're feeling harassed, which hopefully doesn't come up too often, you should be able to do something about it (other than replying, which often makes matters worse). Another exception is flagging your own post, e.g. when it was deleted and you've edited into better shape. You can then use a flag to ask the ♦ moderators to undelete it.
In your case, on another person's question, there's not much you can do about it without a bit of reputation. But don't worry; we have a lot of active users who will notice it too, and know how to use their flags.