Rock Lee has no talent for ninjutsu and genjutsu and only an average talent for taijutsu.
This does not mean that he cannot create or control chakra. In Lee’s first appearance, Sasuke noticed the chakra in his arms, implying that he was enhancing them with chakra to block those kicks and was likely how Lee pulled up a giant tree root. He proves that he can use chakra when he walks on water which makes it almost certain that he can climb trees with chakra too.
So, Lee can convert his stamina into chakra and control his chakra as described for the tree climbing and water walking exercises.
This would mean that Lee’s specific problem is that he has no aptitude for transforming his chakra into a jutsu. He can create and control chakra but not change its shape or nature, even with hand-seals.
This does not mean it is impossible for Lee to learn ninjutsu and genjutsu.
Might Guy suffered from the same lack of talent but has demonstrated the ability to use the Summoning Technique as an adult. And he has dispelled genjutsu which is something Lee should be capable of as it works by disrupting chakra flow and then using more power to cancel the genjutsu, making it closer to chakra control than ninjutsu or genjutsu.
How did Lee graduate from Konoha’s Academy?
If the Academy is willing to accept someone couldn’t use ninjutsu or genjutsu, they should be willing to let them graduate too as long as the student shows enough skill in other areas to be worth it. Guy mentions that Lee “practised his mediocre taijutsu like crazy” before we’re shown Lee after he just graduated, implying that his taijutsu advanced enough that he was able to graduate.
Furthermore, there is the precedent set by Might Guy who was unable to use ninjutsu and genjutsu at the start but eventually became one of Konoha’s best jounin. Thus it’s likely Lee can theoretically learn ninjutsu or genjutsu if he was willing to put in at least as much hard work that he puts into his taijutsu training.
Perhaps Lee makes up for his deficiency by studying ninjutsu and genjutsu zealously. Practising how to detect and counter genjutsu with as much effort as his taijutsu training, learning what every chakra nature is capable of, and memorizing the details of every jutsu he can find so that he can react to any enemy with overwhelming knowledge and violence.
If people knew how much he had really written in that notebook, they’d call him the Second Coming of the Professor.
The Narutoversity 