Although Luke Skywalker is the main protagonist of the Star Wars original trilogy, Yoda actually doubted that he would be able to train him in the ways of the Jedi. Even the most elite of their craft have their doubts, and it appears that Grandmaster Yoda was no exception. Luke Skywalker might have become one of the greatest Jedi and helped his father, Anakin, fulfill his prophecy as the Chosen One, but he was by no means a perfect candidate to begin with.
It's easy to see how Yoda might've been hesitant to take the young Tatooine farm boy as an apprentice in The Empire Strikes Back, especially considering the fact that Luke was too old in the original trilogy to begin Jedi training. While Yoda definitely knew of Luke's hidden potential due to his parentage, he also was aware that potential alone was not enough to make a Jedi Knight. A true Jedi required discipline and tact - two qualities that Yoda believed were lacking in Luke.
Yoda's doubt is apparent in two stories in the book From A Certain Point Of View: The Empire Strikes Back - "The First Lesson" by Jim Zub and "Vergence" by Tracy Deonn. In "The First Lesson," Yoda reveals a lapse in his own judgment, reflecting, "Yoda would try to show him a way toward the light. The old Master hesitated, realizing his own momentary doubt. No, he would not try." This initial feeling of doubt directly goes against the lesson Yoda impresses in The Empire Strikes Back, when he tells the young Skywalker, "Do, or do not. There is no try." This instance supports the idea that Yoda's Force Ghost in Star Wars: The Last Jedi was a better Jedi Master than he'd ever been when he was alive. It's jarring that the 900-year-old mentor strayed from one of his most iconic tenets; this shows that Yoda, for a moment, actually doubted in his own abilities to train Luke Skywalker as a Jedi.
Still, much of the old Jedi Grandmaster's wariness stems from Luke's poor temperament. In "Vergence," Yoda says to Obi-Wan, "And if I try to teach this rash, this impatient, this mindless boy the ways of the Force and fail, what then?" Given this conversation with Obi-Wan, it's clear that Yoda took some serious issue with Luke Skywalker's personality and believed that these traits would impair the would-be Jedi's ability to learn. Still, Yoda's language holds an element of self-doubt; he uses the phrase "try" again, implying that he might not be able to "do" the task at hand. Moreover, Yoda's statement implies that he would be the one failing, not Luke, should Skywalker prove unable to complete his training.
It's possible that these seeds of doubt were planted in Yoda due to his previous experience with Anakin Skywalker. As Anakin's son, Luke inherited many of the negative qualities that turned his father to the dark side. Just as Anakin sometimes lacked the ability to, as Mace Windu noticed in Star Wars: Episode I - The Phantom Menace, be mindful of his feelings, Luke also was a little emotionally reckless to begin with. Acknowledging that Luke was similar to his father, Yoda changed his tactics and ultimately molded his student well, even though Luke didn't train as a Jedi for very long. While Yoda tested Anakin's abilities by quizzing him on images he couldn't see in The Phantom Menace, he tested Luke's character by pretending to be a bumbling swamp-dweller in The Empire Strikes Back. He also took a more hands-on approach to Luke's apprenticeship than he ever did with Anakin's, which prepared Luke to complete his final trial - confronting his father - successfully on his own.
Though Yoda had his doubts with training Luke, it's clear that his mastery helped create one of the most skillful Jedi in the Star Wars franchise. It took more than just the Jedi Grandmaster's ability to, in spite of his own reservations, help mold Luke Skywalker into the next generation of Jedi; it also took a moment of deep self-reflection. That self-awareness solidifies Yoda's legacy as one of the greatest Jedi to have ever lived.