The error is here:
It’s complaining that it doesn’t know what a UUserWidget is. Try adding "include “Blueprint/UserWidget.h” " to the top of your DustyGameMode.h and see if that fixes it.
EDIT: Also, make sure you re-generate your VS Solution, someone may have recently added a new dependency. Just right click on your <Game>.uproject file and select “Generate Visual Studio project files”.