It could be that you made a mistake while entering the key. Please enter the key exactly as it is provided and make sure that you do not confuse 0 with O or 8 with B. It might be easier for you to copy and paste the key instead of typing it manually.
If you have purchased multiple products or multiple product versions, please make sure that you are entering the right key for the right product version. Please note that you can not activate a new major version (upgrade) of a product with your old key even if you are eligible for a free upgrade.