If you bought a cheap key from a website based in Argentina or Russia, but you live in the US or Europe, the key will not work. You need a VPN to activate it (which violates EA’s ToS) or a refund.
If you are in possession of a code and need to redeem it, the process varies by platform. Here is how to handle the activation for the most common scenarios. Need For Speed Hot Pursuit Product Activation Key
Be extremely careful. Third-party sellers often sell "global" or "region-locked" keys. If you bought a cheap key from a
This is the most complex scenario.