Codes are typically tied to a single user account. If you bought a used physical copy, the previous owner may have already registered the key. Why You Should Avoid "Free" Online Keys
Are you having trouble with a during the activation process?
Look at the back of the instruction manual or on a white sticker inside the DVD case.
Keygen software often contains trojans or spyware.
Steam handles activation automatically. You rarely need to enter a code manually, but if prompted, right-click the game in your Library and select "Manage" then "CD Keys."
Here you'll find all collections you've created before.