I'd currently recommend installing the development version on Github (https://github.com/mkearney/rtweet). Without seeing your session info and script, I wouldn't be able to tell you exactly what the problem is. My guess is something went wrong with your API token/oauth (see the Github link for token instructions) or you're missing a dependency. If it's the latter, you could make sure you have "dplyr" installed, but you're better off with the Github version anyway, which only has a few dependencies.
If that doesn't fix the problem, I'd be curious to know what's going on. The transition from twitteR to rtweet has sped bings up a bit, but the next CRAN release should include a few more functions and a lot more documentation and useful error messages. Until then, feel free to post any issues to Github (I'll try to check on here, too, but it probably won't be as frequent).