Sims 4 does not start via steam

@Hemfuda  Try uninstalling the EA App with Revo Uninstaller (the free version is fine), and restart your computer before reinstalling the App.  Run both Steam and the App as an admin: right-click each desktop shortcut and select "Run as administrator."

If this alone doesn't help, I'd suggest uninstalling Steam with Revo as well; even though other Steam non-EA games work fine, the problem could be something Steam uses specifically to link to EA servers.  The problem here is that by default, uninstalling Steam also removes any Steam-installed games, as the game library is within Steam's program files.  So if you don't want this to happen, create a new game library and move your Steam games there first.

If that would be too much of a hassle, you can try uninstalling Sims 4 with Revo instead, and reinstalling into a different folder just to be thorough.  If you do this, and you still have saves or other user data you want to preserve, rename the Sims 4 folder that data is in (it was previously in Documents > Electronic Arts before you moved/deleted it) so that Revo doesn't flag it for deletion.