I cannot activate the key.
Are you activating the game on the right platform?
Please make sure that you're trying to activate your key in the designated platform. For example, Steam keys do not work on Origin, etc. Make sure to follow the Activation guide available on the product page.
Did you enter the key correctly?
When rewriting the key, it's easy to make a mistake if you accidentally swap similar characters--for example, the letter “O” with a number “0”.