"Not an answer" is rather about answers which do not attempt to answer the question: for example "please give more details about your problem" or "I have this problem too, help!" are not answers. Answers which are blatantly off-topic are not answers either, but they will probably be deleted by their author without the need of a moderator.
In your case, you have an answer which is not helpful to you. As said by Dan, the point of SE's format is to have several answers and be able to pick the one you like. Within this format, you have several ways to point out that the answer is not helpful: you can leave a comment if you think that the answer can be modified to be more helpful, or even edit the answer yourself if you feel that it would improve it without defacing it; and if you feel that the answer takes a totally "wrong" direction, you are free to downvote it.
This applies both to a random user reading the question, and to the OP. The OP has one additional way of action, by not accepting the unhelpful solution.
Don't feel too bad about posting a critical comment (nicely), normally people who try to second-guess your XY-problem are aware that they are taking a risk.
Note that none of these actions actually remove the unhelpful answer, because the answer doesn't need to be removed. It may be unhelpful to you because of specifics of your situation, but it may still be helpful to someone having the same problem (without the specifics).
I am not a moderator and I do not know why this flag specifically was rejected. On the other hand, the other answer appears to address this point already. So... please don't flag?