This problem can have several possible causes. The possible causes and necessary steps to solve the problem are as follows:
You have tried to register an old version of the product using a Registration Key for a newer version (e.g. Version 2013 instead of 2011).
Make sure that you have installed the version of the product, for which you have purchased your Registration Key. You can see which version your Registration Key is valid for in your order confirmation e-mail.