Some C&C: Red Alert 3 buyers received a manual with an incomplete CD key. Kotaku writes "a small number" of manuals have an unfortunate misprint, and lack one of the 20 characters required to activate the game.
So what's the solution the online C&C customer support proposes? Just guess, in the worst-case scenario you'll only have to enter the CD key 35 times before you got the right one..
There is currently a work around that may allow you to bypass this issue. Since you have the first 19 characters of the code already, you can basically try "guessing" the last character. To do this, simply enter your existing code, and then for the last character, try the letters A-Z, and then the numbers 0-9. You should eventually get the right combination, and be able to play the game.
In order to get a replacement code, please click the "Contact Us" link on the left side of the page to send an e-mail to our team. If you would like you can also contact us by phone using the number found on page 28 of your manual.