Sometimes things go wrong. Here is what to do if Steam rejects your key:
If you already have the game in your library, Steam will tell you. Note that activating a "Game of the Year" edition key won't usually give you a second copy of the base game if you already own it. Where to Buy Safe Steam Keys
If you aren’t at your main computer but want to ensure your game is ready to download when you get home, you can activate keys via any web browser. Go to the Steam Product Activation page. Log in with your Steam credentials. Enter your code in the box provided.
The short answer is: However, if the game is a "Steamworks" title (meaning it uses Steam for updates and achievements), you can absolutely activate that key.