A common misconception in One Piece is that Ace never possessed Haki.
Ace did use Haki before his untimely death. However, there was never any indication until much later in the story. He uses it blatantly only after the time skip, albeit in flashbacks and light novels. Some fans have theorized that Eiichiro Oda had yet to flesh out Haki as a concept at the time.
Realistically, there is no way a New World pirate could feasibly survive without Haki. As a testament to his power, Ace is one of the rare few characters to possess all three. In the main story, however, he is only seen using the Conqueror's Haki, making use of the other two in supplemental materials.
Does Ace ever use Haki in One Piece?
Ace mainly relies on using his powerful Devil Fruit, the Mera Mera no Mi. He is rarely seen using Haki in combat situations. Of course, he is fully capable of using it in One Piece, and there are a few instances of it in the main story to corroborate it.
He used Conqueror's Haki in his childhood
When Ace was ten years old, he knocked out most of the Bluejam Pirates with his Conqueror's Haki. There is a reason why the World Government truly feared the son of Gol D. Roger, with Ace easily having the potential to become the Pirate King.
However, this only happens in flashbacks to his childhood, and Ace was never shown using it as a young adult. He can demonstrate it with video games like Pirate Warriors 3. Of course, this shouldn't be used as a source of canon for One Piece.
One Piece novel A confirms Ace can use more types of Haki
These One Piece novels outright state that Ace can use both Armament and Observation Haki. Using these abilities, he manages to beat a Vice Admiral. However, the novels also suggest that Ace was too reliant on his Armament Haki.
Of course, this information only shows up in supplementary materials, and as a result, many One Piece fans are completely unaware of Ace and his Haki usage. This causes a lot of misunderstandings within the community.
Here is where the misconception comes from
Armament Haki did not have visual cues before the time skip. Since Ace died beforehand, he never got a chance to showcase these abilities. That doesn't mean he didn't have it, but rather Oda had yet to fully develop the concept.
Haki is used multiple times before the time skip, yet they often lacked visual cues. This mainly applies to the Color of Armament, which is recognizable by a shiny metallic coating. It's arguably the most common type of Haki, and gives characters a significant increase in power.
It's understandable why so many people think Ace didn't use Haki, since there were no visual cues for it at the time. These issues have also led to some confusing moments in his fight with Blackbeard. Ace can't believe it when his Logia abilities were bypassed by the Yami Yami no Mi.
However, anybody with Armament Haki could also do the same. This shouldn't be shocking to a New World pirate. What's even stranger is that Ace doesn't even bother using Haki against Blackbeard, and makes it seem like he has no understanding of it in One Piece.
A common explanation, as stated previously, is that Armament Haki wasn't fully realized as a concept until after the time skip.
Note: This article reflects the writer's personal views.