If you can identify which post (question or another answer) the "answer" should be made a comment on, then it's best if you raise a custom flag, saying:
this should be a comment on https://superuser.com/a/819760
Of course, replace the link with the actual link to the specific target answer. Or, if it should be a comment on the question:
this should be a comment on the question
This will make it very easy for us to handle it. In general, when flagging, tell us what you want us to do, rather than just flagging as "not an answer" (which would require us to check the contents, find the target post, etc.)
Feel free to add a reason why you think it should be made a comment rather than just deleted.
And, don't forget to add a comment yourself, saying something like this:
This is really a comment, not an answer. With a bit more rep, you will be able to post comments. I'm flagging this post for conversion to a comment.