If you hit a 403. That is, the request if forbidden for some reason.
Read more here: https://en.wikipedia.org/wiki/HTTP_403
It could mean that the server is suspicious because you don't have a proper user-agent in your request, in wpscan you can solve this by inserting --random-agent.
You can of course also define a specific agent if you want that. But random-agent is pretty convenient.
