sims 4 won't launch

Try this: go to The Sims 4 in your Steam library, then click the Manage button (settings icon), followed by Properties, and then Installed Files. From there, click verify integrity of game files. Try to reopen after that and see if it works. If not, you may need to do a fresh installation of the Sims 4; make sure to back up your saves!

Sims 4 > Manage (settings) > Properties > Installed Files > Verify Integrity > Launch

Also, try to clear the cache and other files before re-launching. Goodluck!
Don't hesitate to reply if the error persists. I'll try my best to help you. 😊

Steam - Verify Integrity of Game Files
Clearing Cache Guide
Backing Up Your Saves