One of the quests was to complete a Nature aspiration goal. So if your sim doesn't have the Nature aspiration I am assuming you must change your sim's Aspiration to complete that goal? Not sure exactly how I go about doing that but is there a potion you have to drink (or your sim has to drink) to change Aspirations under the Rewards store?
When you open the aspiration panel, click on the star icon that is on the top right hand side. Then that will open the section where you can choose a new aspiration. For the event task, choose an aspiration from the Nature section, then confirm your selection. Your sim will then be able to do the new aspiration.
