Have you already created an account with that key in the past?
These keys are generally single use and will remain tied to the account you redeemed them on originally.
Could you have mistyped the Serial key? Some keys have dashes that are required as part of the key and some are case-sensitive.
Please re-enter the information, or even better Copy and paste the key into the activation field using CTRL+C and CTRL+V
Do You have a firewall, router, ad blocker, pop up stopper, or another program that could be blocking the account server from identifying your key?
It needs a clear connection to you to authenticate the key, make sure you have added your game's exe file to your firewall software as an exception/permission. You can see a list of guides for popular protective software Here
Keys are provided only to the purchaser's email address and are not duplicated or invalid. Direct2Drive receives all our keys directly from the game's publisher, and does not deal in resold or second hand keys.
If you feel none of the conditions above apply, please contact the site on which you are attempting to register the key and inquire on the use of the key from their Support team. Because Direct2Drive does not have access to your MMO’s account system we are unable to troubleshoot activation for MMO titles. We recommend contacting the publisher for further assistance!
To locate publisher support information, check your Start Menu by the game title or the game’s documentation or see our FAQ How do I contact publisher support.