Serial keys for Shift 2: Unleashed are one-time use. If you bought a used physical copy, the key is likely already "bound" to the original owner's EA account.
can be tricky because the game uses a specific DRM (Digital Rights Management) system that limits the number of times a key can be activated. Where to Get a Valid Key serial key for nfs shift 2 unleashed
Shift 2 - Unleashed - Xbox 360 (Limited) : Video Games - Amazon.com Serial keys for Shift 2: Unleashed are one-time use