Had two open mana in the form of treasure tokens with a Bitter Triumph in had, 6 life, and three total cards in hand. Put a stop on the opponent's upkeep to target their Voice of Victory and I couldn't cast it. Figured I might have to wait for them to draw or take an action to be able to cast it in response so I passed to draw and still wasn't able to cast it. Opponent animated a land and proceeded to attack. Wasn't able to cast Bitter Triumph at any time up to this point. Tried sacrificing both treasures to see if that would allow me to cast it and still nothing. Ended up losing the match because I wasn't able to cast an instant spell when I should have been.