Why can't I pick a "best answer"?

You have to wait for 4 hours so that people can see the question because if you picked best answer after 4 hours a blue on that says choose as best answer will appear pick the answer that you liked Once you choose a question and when u do u gain 3 points And the other person that got picked for best answer gains ten If you do not pick your best answer in 4 days it goes into voting and it will be chosen by voters