Obi-Wan Kenobi is a classic Star Wars character whose backstory and legacy reach far and wide, but what is his weakness after the prequels? Star Wars characters don't have weaknesses like conventional superheroes and similar figures do. Weaknesses in Star Wars are more so character defects or idiosyncrasies that keep the heroes and villains from moving forward in their arcs.
Darth Vader's weakness, for example, is his past and how he loses his head when something near and dear to him presents itself. In Obi-Wan Kenobi, he loses sight of his main focus, going after the Empire and destroying the rebellion, in favor of hunting down Obi-Wan himself and fulfilling what he needs to get closure. The fights between them in the series feature Darth Vader not only letting Obi-Wan live to make him suffer but ignoring the Jedi Rebels and Force-sensitive citizens' escape in order to kill Reva, the surviving Jedi youngling. Darth Vader's weakness is, therefore, part of his character and not a solid point of physical vulnerability like kryptonite is for Superman. Weaknesses aren't necessarily things that are "wrong" with the characters, but whatever it is about them that seems to stand in the way of their ability to move forward.
When looking at Obi-Wan specifically, all throughout the prequels and the show, his fear gets the best of him. Prior to A New Hope, Obi-Wan Kenobi faces and must quickly get over multiple traumatic events. He had to watch his master, Qui-Gon Jinn, die a gruesome death at the hands of Darth Maul and was then forced to immediately avenge him. He fought in the Clone Wars and lived through Order 66, witnessing the very extinction of his own people, and surviving to tell the tale. Later on, Obi-Wan had to watch his best friend and pupil Anakin Skywalker turn to the dark side. He also had to live with the fact he basically trained Darth Vader and watched him turn on the Jedi, killing countless people even after Order 66. Given the events of Obi-Wan Kenobi and the prequels, it is pretty clear that Obi-Wan's ultimate weakness is likely his PTSD.
Obi-Wan Kenobi Has PTSD And The Story Is Better For It
After living through all of this, Obi-Wan Kenobi went into hiding on Tatooine, becoming a hermit and a recluse, living out his days alone in his tiny house in the mountains. In the events leading up to A New Hope in Obi-Wan Kenobi, he got a dead-end job helping to package meat from a huge creature, had his wages stolen by the foreman, and simply accepted this as his fate. His dissociation in the dunes of Tatooine marks a very common symptom of PTSD and demonstrates why he hadn't used the Force in so long.
Obi-Wan left his past behind to hide and protect Luke Skywalker, but he had to forget his own identity to survive without being caught. This dissociation, along with his trouble concentrating, overwhelming guilt and shame, and his always being on guard for danger, can all be symptoms of PTSD. PTSD was first diagnosed in 1980, three years after the release of the first Star Wars movie, so this connection wasn't intentional then. But in Obi-Wan Kenobi, it comes to a head because the character has to dig himself out of his rut in order to face and defeat Darth Vader.
Obi-Wan heroically conquers his fear at the end of the series, finally being able to commune with Qui-Gon Jinn's Force Ghost. However, his PTSD seems to recur in his life after the series and before A New Hope. By the time the events of the original Star Wars movie come around, Obi-Wan Kenobi is still a recluse not practicing the Force, and whenever Luke asks him about his past, he looks off into the distance, pensive and pained. Ultimately, these decisions make the character's journey more believable for viewers and capture the very real trauma he's endured.