The game does precisely that: "A new Special Task has been added to the Quest Board" should have flashed up on the screen.
vvv887, Humans aren't an option because there's only one human in the world. And some of Falene's dialogue tells you that the Nephelym half is always dominant in cross breeding that involves humans.
It's possible that humans died out in mass crossbreeding events. This is why Falene and almost everyone else is so surprised to see one as in the world this game takes place in, humans are thought to be nothing but myth.
But outside of game lore, Humans are a race of precisely one person. Which essentially means that Humans aren't actually a race. So you can't select a non-race as a race.