I have already finished the SAVE THE LAST DANCE quest. The quest still appears "Place the Wall mirror". I have already placed it and finished the quest, but it is still there.
This sounds like a bug. Could you open a new post in the bug forum for this? It would be great if you can add some screenshots of what you're seeing in-game.
You can find the bug forum over here: https://answers.ea.com/t5/Bug-Reports/bd-p/the-sims-mobile-bug-reports-en
The bug forum has a small list with questions that might just help to solve the problem!
I do not work for EA