If you can't find it right after finishing the goal, try force closing your app then reopening right away. Sometimes the prizes I win don't show until I restart my app like that.
I can't say exactly where it is in your inventory since item placement changes based on the other decos you own. But in general, "Halloween Scarecrow" should be toward the front of your inventory, with any other Mystery Box items you own, not in the back with the goal items. In my game the deco is after the "Persian Cat" but before the "Harp Player."
If, after restarting your app and double checking, you're still unable to locate the deco (you think you weren't rewarded the prize) you can open a
Support Ticket with the team who should be able to assist you.
Good luck!