A half elf player with the psychic power talent is possible to cast a spell without throwing any dice?. I guess not, because you have to spend at least one willpower and it counts as two. Is it correct?
That is correct, you have to spend at least one WP. Psychic Power means that the Half-Elf can count the first WP as two (and if he spends two WP, they count as three, total). But do remember that if he chooses to use his Psychic Power (you are never forced to use a talent), this will also increase the risk of a mishap.
I assumed that being a racial talent it was always applied, but I see that it doesn't make sense because there are other racial talents that are applied circumstantially