I am writing a science-fantasy book series for adolescents (or people aged from 10 to 24 years old if you want) about a weird creature called Céleste Bizarre. One of their friends is an anthropomorphic human-sized flightless falcon called Félicie Faucon (the names sound French because this is my native language). But, I wonder how could a falcon species evolve to be flightless. I mean there is a species called the peregrine falcon that wins the Guinness World Record of the fastest flying animal (and the fastest animal overall). I also know that falcons belong to the Falconiformes order, and their closest relatives are seriamas, parrots, and passerines (Cariamiformes, Pstittaciformes, and Passeriformes, respectively). Together, they form the Australaves grand-order, and the only living flightless Australaves (Telluraves?) species is the kakapo. However, there was once a flightless Falconiformes species: the Jamaican caracara (Caracara tellustris).
The big point to consider is why flightlessness evolves. Flight requires an absolutely ridiculous amount of energy, so if there's a way to do without it easily then boom, flightlessness. This usually happens on islands, for reasons I'll get to shortly. Flock of birds gets blown to an island, where there are no predators. They can't get back because they got blown way off course, but there's plenty of (ground-accessible) food and no predators/competition. Importantly, because it is an island, it will stay isolated. That is why New Zealand, pre-colonization, was absolutely dripping with flightless birds (including the kakapo). Same thing on Mauritius, with the Dodo, &c.
Long story short, have a few falcons end up on an island with lots of easy ground-based food sources, and give them a million years or so without outside interference. Or posit an undiscovered island where this has been happening over the past million years; I'm not to optimistic about humans still kicking about a million years from today.