When creating a new action if I edit the "Fill Opacity" on any of the layers, later when I play the action on another image it says "The command “Set” is not currently available." and it does not add only the "Fill opacity" which I know I added in the original action.
»Photoshop CS6 Action Bug.«
What gave you that idea?
What is selected when you start running the Action and what was selected when you recorded it?
What are the exact steps of the Action, could you please post an screenshot of the expanded Action in the Actions Panel?