Warlock Quest is to take life from another player, but I'm not allowed to attack them. Can I complete the quest?
If the particular game scenario (such as Lightbearers) prevents any player-player combat, then the quest is never achievable, and the quest should be discarded and another one picked instead. However, if there is a possibility that a character could die and be replaced by another that could be attacked, then the quest should be retained against that possibility. That would be one of the risks of taking a Warlock Quest! Of course, your group could decide together to discard any unachievable quests and pick another.