The game wont launch on Steam Deck

I'm afraid you were misled into believing that SIMS4 was compatible with Steam Deck. Here's the problem:
Steam Deck runs Linux. You can install the EA app and play EA games on Linux, right up until EA update the EA App. Then it breaks. The EA app will not update under Linux.  EA have recently updated their app and I am facing the exact same problem on my desktop Linux as you are facing on the Steam Deck. EA refuse to make the EA app work properly under Linux. Steam claim that EA games will play on Steam Deck, which they do - perfectly - but as soon as EA update their app, nothing works. I don't know what the answer is here. EA hate Linux, it seems they don't care if people cannot play EA games on Steam deck and, worse, imo they don't care if people are spending hard earned cash on a device that will not work the second the EA app is updated. I opened a thread about this very issue less than an hour ago and EA's only response was to silently bury the post in a thread that was five years old and had been closed! The only thing you can do, probably, is raise holy hell with Valve, they claim Sims4 runs on the deck - and it does - but they fail to point out that it will stop working the moment EA update their app. Essentially you were mis-sold the device imo. I don't think people are really aware of this so help spread the word if you can!!