Try rebooting your device, check your main inventory not a category,
If you have an account, uninstall completely, clear 6-8 gbs of free space reboot and reinstall, do not play while downloading.
You might try EA Help on Twitter m-f US central time business hours https://twitter.com/EAHelp
You will have to open a ticket with EA directly through the in game link or on Twitter
Unfortunately if the new update and install does not help, only EA can help you.
As I said: I finished the star collecting before I updated the app (usually it doesn’t let me play unless I update, when an update is available). No reward was shown. When I updated the app, it showed the prize building and the text that I had received the award.
I have, in the meantime, removed the app and reinstalled it, to no avail.
One question ... does the questline image look like this to you too, with the missing building? (the image is from an Italian-speaking user)
You will have to be persistent and get EA to help you. If they tell you to post on the forum tell them you have and they told you EA had to help you.
The House and the quest were not loaded into the game until the Halloween update so if you did the star tasks and then updated, it had nothing to give you. You must get EA to help you. Contact them by phone or chat is best or Twitter M-F US Central Time business hours.
Let this be a lesson: Always update ASAP when a new one is available!
Check the Game Discussion Forum daily for updates about the game. It's almost always announced there a day ahead. Also, any glitches or problems with the game are complained about almost instantly.