Sims 4 (steam) does not open.

Atlooos  Having two user accounts is not an issue, but not fully closing out Steam and/or the EA App in one account, then trying to use the other, might be.  So I'd switch accounts, open both apps, use the Exit option (rather than Xing them out) to close them, and kill any related process in the Task Manager.  Then go back to the other account and try again.

Another potential problem is OneDrive, so please pause syncing if it's running at all.  Right-click the cloud icon in the lower-right corner of the screen, which might be visible or might be hidden in the Tray (^ icon).

Jehane40  EA games don't support Steam family sharing, and in fact that is one reason an EA game might not start through Steam.  So Atlooos if you are using family sharing, please disable it.