When the "Roll" Button is shown, you select the dice to roll in the bottom right corner. I believe the games says bottom left which is a mishap from our side.
There should by default be a square with a six in it, which is the six-sided die. You always get a six-sided-die at turn-start if you don't have one. You can find other dice throughout the game to roll with tho.
If clicking on the dice in the bottom right corner doesn't do anything, then that's a bug and we're sorry.