The NAA flag is meant to be used when one encounters a post that is Not An Answer. As the name implies, it's not meant for wrong answers, as those are still legitimate answers.
If used properly, one will find their most common use of the NAA flag will be for answer posts that:
- Ask a new question
- Seek more information from the OP without providing him a solution
- Thank another poster
In my experience, much less frequently than these scenarios will be cases where the OP asks What is the first day of the week? and the answer is, "You need version 15 of the compiler." This isn't a wrong answer, it's an entirely unrelated post, hence it being appropriate in this case to use the NAA flag. As noted in the comment by @Kate Gregory, leave a comment on the answer before flagging it explaining the problem to the poster and those reviewing the flag.