Welcome to the forum. :) It may be that your game did not synchronise, so you have the quest again. A good tip is to go to friends screen and back to your town. When you return to your game you should see the word ‘synchronising’ near the bottom of your screen, meaning that gameplay has been saved.
There are lots of tips for the game in this thread:
https://forums.ea.com/en/the-simpsons-tapped-out/discussion/90074/tips-for-new-players/p1