I need 5 digits serial code.

There are two different kinds of codes:

16 digit ones are for limited edition Store content. Those are sent out by email and can be redeeemed here: https://store.thesims3.com/myAccount.html and then you'll find the item in your thesims3.com purchase history to download to your game: https://store.thesims3.com/purchaseHistory.html

20 digit ones: Those are the product or serial codes which you need to install the game and they can be found on your Origin client

Right click on the game and choose Show game details. The code should be there

If it's not there, log out and back in and/or restart Origin