Voldemort is undoubtedly the greatest dark wizard of all time, but there is one major spell he couldn’t cast in Harry Potter. Although often compared to Grindlewald in terms of their power, Voldemort’s skill is definitely superior due to the lengths he would go to for his own selfish gain, so much so, that only an evil Dumbledore could be more powerful than Voldemort. Despite this, Voldemort can still be overpowered and a lot of light magic was successful against him, such as Harry’s mother Lily being able to stop him with love and its ancient magic.
By the end of Harry Potter, the Horcruxes had caused Voldemort to barely possess a soul, although this did not impede his powers. The whole Harry Potter series saw him grow in strength and use his dark magic to increase his powers, despite his soul being split among many objects. The dark magic allowed Voldemort to master magic that was wildly shunned, even speaking Parseltongue was directly related to being evil, but there were certain aspects of magic that Voldemort was unable to master despite his power.
One major spell Voldemort could not cast in the Harry Potter franchise was the Patronus charm which is a different shape for each character. The complexity of a Patronus charm should not be an issue for a skilled wizard such as Voldemort, however, the spell is dependent on the caster’s positive emotions and memories for successful use. Voldemort is shown to feel happiness when he is linked to Harry in Harry Potter and the Order of the Phoenix Harry feels that Voldemort is happy and pleased about something. Even though Voldemort’s soul was fragmented it does not seem to have an effect on Patronus charms and Voldemort’s lack of emotions isn’t a reason that he cannot cast a Patronus however, only those who are worthy can produce a Patronus. This is also the reason that Snape is the only Death Eater, able to cast a Patronus charm because, as a double agent, his intentions are pure which is clear through the clues Snape isn't a villain.
Voldemort Didn’t Need To Be Able To Cast A Patronus Charm
One unworthy person who successfully cast a Patronus charm was Dolores Umbridge. In Harry Potter and the Deathly Hallows, Umbridge’s corporeal Patronus, formed as a cat, holds off Dementors when she questions muggle-born Wizards and Witches in the Ministry of Magic. However, Umbridge is wearing the Horcrux locket at this point and, according to JK Rowling (via Stack Exchange), her evil affinity with the locket helped her magical ability and enabled her to produce the Patronus while deriving sadistic joy from her work, whereas, the locket hindered Harry’s ability to produce one.
This reasoning means that Voldemort and his Death Eaters could potentially learn to cast a Patronus but, particularly for Voldemort, he had no reason to. From his rise to power, Voldemort is shown to have control over the Dementors, who hurt Harry particularly badly, at Azkaban and then throughout the Battle of Hogwarts. Voldemort doesn’t have a full soul for the Dementors to take but the Death Eaters do which further confirms his control over them. Voldemort’s affinity for dark magic means he is not a threat to the Dementors and is why he can control them, so even though he does not produce a Patronus in the Harry Potter franchise, it does not mean that he cannot.