I don't think it's a contradiction. Yoda is saying there is nothing more he can teach him, but that he still needs to confront Vader to graduate; confronting Vader isn't something Yoda can provide so this is something Luke needs to seek out. I think in fighting Vader in ESB, and learning from that and meditating on it, and practicing in the months since then, he is in the place that Yoda wished he was at the end of ESB--capable of taking on Vader, emotionally and physically--and now he just needs to go ahead and finally do it.