I believe the current button to trigger a challenge is the ESC key, it should work on most NPCs but not all will have duels attached to them.
We'll be making the challenge system a bit more intuitive in the next update.
So in the card beasts tournament in Goldeka, after beating the first opponent the game freezes. The clown guy says something and then the girl turns to the wall and it freezes. I can't move, I can't right click or do anything except close the game and reload from right before the battle which the same thing just happens again. If it's a glitch causing an error or a crash that would suck but I figured I'd ask if there's something I'm supposed to click maybe? (I dunno I'm grasping at straws luckily I have a save not too far back though).