You must buy every standard upgrade and choose every store on a floor before the Set Piece GUI becomes available.
For a look at how these Set Pieces and their corresponding GUIs function during end-game play, check out this completion walkthrough: Mall Tycoon GUI
Use the GUI to let players select their shop type before the script places the model. You must buy every standard upgrade and choose
Set Pieces are unique structures located at the end of every floor. They are more expensive than regular stores (usually costing 30% more) but provide significant income through "Mega Tips". Mall Tycoon GUI