If you have not done so already, complete Steps 1-4 of the GameplayAbilities API – triggering an actor's gameplay abilities with game controls recipe to link to the GameplayTasks API in your ProjectName.Build.cs file and enable its functionality.
Afterwards, go in the UE4 Editor and navigate to Class Viewer by going to Window | Developer Tools | Class Viewer. Under Filters, uncheck the Actors Only and Placeable Only options.
Ensure that the GameplayTask object type exists:
