Most manga/anime have their fair sharing of oddball characters. For Naruto, that character is Might Guy, along with his disciple, Rock Lee. While almost every character in the series showcases their considerable skill in ninjutsu, there are a handful of exceptions. This includes Might Guy and, by extension, Rock Lee. However, it's not that Might Guy can't use ninjutsu, it's that he chooses not to. And there might be a very compelling reason for that.
Right from his youth, Might Guy was known for his inability to use ninjutsu, a trait he seems to have inherited from his father, Might Duy. Duy spent his entire life as a genin because he couldn’t get into the academy as a result of his inability to use ninjutsu, and his son almost found himself in the same situation. But, with hard work and encouraging words from his father, Guy was able to get in.
Guy has a little talent in ninjutsu, and in fact, his chakra nature type was confirmed to be Fire and Lightning release. Due to his limited use of ninjutsu and genjutsu, though, he trained himself to become one of the best taijutsu users in Konoha.
Early into Naruto, Guy discovered an interesting trait about Rock Lee: he lacks the ability to use chakra or become a ninja. Fortunately, Guy empathized enough to train Lee into a fine taijutsu user. Throughout the series, their master/student relationship continued to grow stronger as both characters shared a similar taste in fashion and a similar sense of humor.
But Guy's not quite as useless as you might think. He proved he could use ninjutsu to a certain extent by easily making a summoning jutsu -- a jutsu that requires its user to have a considerable amount of chakra to summon a huge animal. As not every shinobi in Naruto universe has a summoning jutsu, this could indicate how Guy’s ninjutsu level might be better than some of the average chunin-level shinobi. He was knowledgeable enough to recognize the Four Red Yang Formation barrier and how hard it was to create such a barrier, and he was also able to dispel a high-level genjutsu.
However, the real reason Guy has refused to keep using ninjutsu-based attacks might not necessarily be because he has poor ninjutsu or genjutsu skills. If you go back to the early part of the Naruto series, Guy compared himself to the pitiful Lee and promised to make him a very strong shinobi even though the latter lacked chakra. As a result, one could interpret that Guy decided not to use ninjutsu-based attacks in battle because of his beloved subordinate. Instead, he wanted to lay a good path and lead his student by example. He achieves this by fighting without ninjutsu so as to show his disciple not to give up his dream.
Guy’s decision might have little effect on the outcome of his confrontations considering he wasn’t that perfect in ninjutsu to begin with. But at the same time, if he had continued to use ninjutsu then he might have improved greatly. Therefore, he sacrificed this opportunity for Rock Lee. Rock Lee also responded positively to his mentor’s behavior by making Guy his role model in everything. He adored Guy so much, in fact, that he cried when Guy opened the Eight Gate (Night Guy) to fight Sage of Six Paths Madara.
Guy’s weakness in ninjutsu didn’t signify that he couldn’t use any at all. He just preferred not to use ninjutsu as a way of showing Rock Lee that he could be a great ninja too if he just worked hard enough.