Please be sure you are you are inserting all the cd-key characters correctly, this happens at times by using look a like characters (Example: "0", "O", "Q").

If you are certain that the code is correctly typed, check that you are using a supported platform. You can learn which platforms are compatible at the product's description page. 

In case you are trying to redeem a Steam code, you may also try the following recommend procedure.

If you are still having problems, please submit a support ticket with screenshots detailing the activation procedure (be as thorough as possible).