sims 4 stuck in windowed mode, won't apply change to fullscreen

@LeonorSabine  Are you using GeForce Experience, and have you let it optimize your game settings?  If you have an AMD card, I think its control panel also offers the option to optimize settings, but I can't remember off the top of my head what it's called.  Anyway, if you need help with this part, let me know what kind of graphics card your computer has.