Found it. I'm posting below, so that the thread will flag with an unread message for you. There used to be an option for it, but I think it was deprecated long ago. AFAIK, you will need to build it into your rolls. You will need to turn on show roll modifiers in your settings. Anywhere you would roll a d20, add <1. I.e. 1d20<1, for re-roll 1's. Kryx might be able to help more. There might be a better way.