The game wont launch on Steam Deck

That's probably Steam.
The game through Steam works in a weird way, on the PC.  When you click play, first it opens the Steam console program, runs some functions, then it opens the EA App with a command routine and then runs the game through EA app and not Steam at all- as far as I can tell anyway.
Point being, you might need an update for your Deck or it's PROBABLY a glitch in Steam.

And please note: I have never used a Steam Deck in my life, I'm guessing ALL of this and I'm sorry if this is a waste of time or if it breaks your Deck.

Backup ALL of your game saves, ideally in 2 places, plus the Cloud Save.

Try running the game directly through the EA app, if you can.

Try updating the device - Settings -> System -> Software Update.

Check if the games are alright:  Settings -> Installed Files -> Verify integrity of game files.

Try running the game in different modes?  🤷‍♂️

Try disabling Cloud Saves and running the game (ya never know!).

If it still doesn't work, immediately turn on Cloud Save, because you might need it for the next few things.

Try clearing the download cache:  Switch to Desktop Mode, open Steam, Settings -> Downloads -> Clear Download Cache.

You might be out of space on the Deck's SSD drive (or your SD card) and it needs memory to run the game?  So try uninstalling a different game you're not playing.

Try reinstalling the EA app and the game.

Something technical:
Switch to the desktop mode -> Run Steam from terminal:
Open Konsole application.
Type steam and hit enter.
Try launching Sims.
It SHOULD send out a bunch of text that will not make any sense or very little, copy and paste that to me here and I MIGHT be able to figure out something - OR it may not print anything and it's already running.  😄

Now for the scary fix attempts...  Please note, these might break the game, but it sounds like it's already broken, so... 🤷‍♂️
Do this as a last resort because it might break some things, including all of your other games... and I've never done it.  😬
It will probably have an effect on ALL of your games, not just Sims.
First, backup all of your game saves!
In Desktop mode, open the folder, access the menu and set the folder to show hidden folders.
Navigate to:  home -> deck local -> share -> Steam -> steamapps
Copy "shadercache" and "compatdata" to an SD card (this is your parachute if things go bad).
Delete the "shadercache" and "compatdata".  😬
Do NOT empty your trash bin (second parachute.  😄😄😄).
Try running the game, again.

Worst case scenario:  Full blown reset of your Steam Deck.  😭