Unlike on some other sites, there is no single way to communicate with a person you have interacted with. They will be notified about:
- comments by anyone (including you) on their questions and their answers. So if by "answered me" you mean wrote an answer to your question, the way to reach them is to comment on that answer. You can do that no matter what your reputation is
- comments that mention them with @username in a thread they are part of. So if by "answered me" you mean wrote a comment on your question or answer, or wrote a comment on something you also commented on, then the way to reach them is to add another comment on that question or answer that @mentions them. This requires a minimal rep if the question isn't yours.
Sometimes users try to @mention other users when those other users will be notified no matter what - when this happens the @mention is stripped out. This is not a bug.