If the code is invalid, you should check if you have misspelt the code. (0 O,5 S,I l or any other similar characters)
If the code has been redeemed already and you have not activated the key on another PC before for sure,
you should contact the EA support at https://help.ea.com/ . Click on "Contact us", enter a question, click on "Find solutions", enter the product, issue type and platform, click "Find solutions", click "I still need help". Now you should be able to chat with the EA support or call them under the given phone number.
You will probably have to prove that you have bought the game. Then you will get a new product key.
(If the EA support is not able to help you which is really unlikely, especially if you are chatting with them, then you should contact the shop where you have bought the game and try to get your money back. In that case, the shop provided you with a wrong serial number.)
