During the fight with Enel on Sky Island, the four priests, including Enel used mantra. Later on, we learn that Mantra is actually Haki.
Luffy trained with Rayleigh to learn Haki. While Luffy was born with it, he wasn't able to use it perfectly. However, at sky island there was a kid called Aisa who used Haki to to sense people from distance. She wasn't able to perform Conqueror's Haki since she was a child, but she was able to use Observation Haki since birth. How is this possible when even Luffy couldn't do that?