"NO! Try not! Do or do not. There is no try."
It's clear that Yoda is not telling Luke to try and try again to get better, i.e. training. He really does say "Do it now or don't even try."
Luke tries and fails and tells Yoda: "You want the impossible."
And then when he sees the X-Wing floating by him, Luke says: "I can't believe it!" And Yoda replies: "That is why you fail."
Not "You are not training hard enough." But: "You fail because you don't believe you can do it."