According to the Command & Conquer customer support team, a "small number of [Command & Conquer: Red Alert 3]manuals" suffered from an unfortunate misprint. Some owners of the Windows PC version are finding themselves with a CD key just 19 characters in length, exactly one shy of the required 20 characters required to activate the game. Oops!
Fortunately, EA has a helpful workaround to help gamers in need. Try to guess what it is.
No, that's actually the solution. Customer support suggests "guessing" the missing character. Start with "A." If that doesn't work, try "B." No? "C" then? Keep going. "D" is next. If it's not a letter, it might be a number. Try those too. The good news is that three dozen failed attempts to activate your game won't lock you out of your copy of the game.
From the official C&C customer support page.
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.
We're guessing at least one of the 259 of 1540 people who claim they found this answer helpful are lying.
Gamers alphanumerically challenged can always contact EA via its online "contact us" form or by good old fashioned telephone.