Re: I'm asked to do an adventure I've already done
@BillFlowerpot The game is supposed to block sims from receiving quests they've already completed, but that doesn't always work correctly. In this case, if your sim has in fact finished the last part of the quest and the game does recognize it, that could be the reason it can't complete again. Regardless, there's nothing else to do here except cancel the quest and find another.
In the future, if an earlier step of a quest is stuck, you can use NRaas DebugEnabler to grant the next step and therefore force-complete the stuck one. But I'd suggest just not accepting any quest that your sim has already completed unless it's one of those simple deliver-these-items options that are supposed to be repeatable.