June 2016 - last edited June 2016
I downloaded Sims 4 back in July and played it for a few months. I decided to jump back into it so I reinstalled Origin and pressed the Sims 4 button on the Start Menu but I was greeted with a box saying I need a code to activate it. I didn't have to do this back in December when I last played it and don't have a code. And yes I've checked my email.
June 2016
You shouldn't have to re-activate The Sims 4.
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When starting my game from the shortcut, the game asks for my serial code again/asks me to activate the game
This usually happens if you are logged out of Origin for some reason or if there was an Origin Client update. Don't use the shortcut to start the game if this happens but log into Origin first (use your Email address) and check if the game still properly shows up in the My Games tab within the Origin Client. Do any necessary Origin and Game updates and then hit PLAY in Origin to play the game.
Source: http://answers.ea.com/t5/The-Sims-4/FAQ-Serial-Codes-Game-Activation-and-Download-Issues/td-p/422823...