PROBLEM - You are trying to install an expansion pack for a game (C&C Generals) without the game being installed. You must first buy and install the game Command & Conquer: Generals, before attempting to install this expansion pack, wich is more of an add-on for the game, even though it is pretty much as big as the original game and acts like one.
Next time, u should check to see if ur buying an expansion pack or the original game. It may be difficult because its not boldly written. A guidline would be to check, if u see that the name of the game is long.