I am playing on PC. I have played several brawl games, where I am playing with Vivi as my commander, and have omnicience on the battlefield. As I may play spells without paying their mana costs. However; Finale of Revelation, I had the mana to pay for 10 or more, but it only casted it, as if omnicience wouldn't allow me to tap mana. I was never given the option to pay for the mana cost.