The Canonical Reason Darth Vader Can't Use Force Lightning Makes Too Much Sense
At this point, it has become clear that Darth Vader (voiced by James Earl Jones) is one of the most powerful and terrifying villains of all time. He was scary enough in the original trilogy, but ever since his savage display of power in "Rogue One: A Star Wars Story," Disney has seemingly made it a priority to showcase his immense strength in each successive appearance. Consider, for example, Vader's appearance in "Obi-Wan Kenobi" — where we watch him easily dispatch a Sith Inquisitor without ever touching his lightsaber, pull down a cargo ship with a single hand (a power flex if we've ever seen one), and massacre an entire village without ever breaking a sweat.
For most people (with the obvious exception of his former master), to battle Darth Vader is essentially a self-inflicted death sentence. His complete mastery of lightsaber combat is rivaled only by his own vast knowledge of the Force — and on the basis of brute strength alone, he is completely unmatched. Yet, confusingly, there is one aspect of the force which Vader seems completely unable to use: Force lightning.
As the most powerful combatant in all of "Star Wars" canon, one might assume that Vader ought to be able to use such a power with relative ease. However, when we look outside the films, it has actually been confirmed, fairly definitively, there's actually a pretty important canonical reason why he is unable to wield Force lightning.
Vader can't use Force lightning because his robotic limbs are not connected to the Force
In Marvel's "Darth Vader" #8 (part of a canon comic book series published in 2017) we witness Darth Vader's true form, as seen by the Force, while he is meditating. His entire torso is a roiling mix of blood-red energy, but his robotic limbs are completely untouched by the force — that is, they appear as white outlines with nothing in them at all. This depiction subtly clarifies the true reason why Vader is unable to use Force lightning: because his arms and fingertips are not connected to the Force.
Although we know Vader can certainly wield the Force through his robotic limbs, this comic makes it clear that the Force itself does not reside within those limbs, and as such it's likely that Vader is unable to channel Force lightning through his arms and hands.
Furthermore, as we learn in "Return of the Jedi," Force lightning has devastating effects on the cybernetics in Darth Vader's body — indeed, a short exposure to the lightning of Emperor Palpatine (Ian McDiarmid) proved fatal. Considering the fact that Vader's limbs are not connected to the force, Vader would simply be channeling lightning through a pair of metal arms, most likely frying them, and possibly killing himself in the process.
To be fair, Vader actually does wield Force lightning in Marvel's "Darth Vader" #25, but explicitly does so without his hands — controlling it remotely outside of his own body. In any case, it's made abundantly clear that Vader's limitations are not a question of power, but a result of the damage he endured in that fateful battle on Mustafar, arguably the most pivotal relationship in all of Obi-Wan and Darth Vader's history.