The problem is that you can't start your tournament until you have at least 2 people bought in. Part of the reason behind that is that, with less than 2 people in the tournament, the software tends to think the tournament is over (if it has started).
So, the reason you can't start the countdown is that at the end of the countdown the tournament starts automatically, but you can't start the tournament without at least 2 people, and what happens if there aren't 2 people bought in when the countdown ends? See the logic?
But, here's a psuedo-bug you can exploit: Add two players to your tournament, buy them in, start the countdown, then remove the players from the tournament. Now buy your players in as the countdown commences.
For confirmation of the problem, set your countdown to 10 seconds, then remove the 2 players so there are none in the tournament and see what happens: when the countdown ends, the tournament starts, round 1 commences, and then the tournament immediately ends.