@Klaudiadrzewo96 The serial code should appear in your game library, under Sims 3 > Manage > View properties.
Having said that, you don't need the code to register with the official site, and the code wouldn't work anyway—it would read as already used, which it is. Instead, if you sign into the Sims 3 site with your existing EA email address and password, the site should know you own the base game already.
If this isn't the case, let me know. You might need an EA customer support rep to remove and re-add the game to your account.
