@sne888 An overlay is a user interface from one program that is superimposed (often transparently or only in one area) on another program currently running. The idea is that you might want ot use some feature of the program providing the overlay without closing the other program. For example, you might be playing a game when one of your friends comes online, and you can use the EA App or Steam overlay to message their friend while still looking at your game.
Many apps designed to be used alongside other apps come with overlays. Most game launchers have one, as does Discord, as does Nvidia's GeForce Experience.
The point here is that overlays can conflict with games at times, in various ways. The suggestion in this thread was based on the idea that an overlay might be active but the player didn't notice or couldn't see it, so when they went to type in-game, they were really typing in the overlay instead without knowing.
Having said all that, if disabling your overlays doesn't help, please open Documents > Electronic Arts > The Sims 4 > ConfigOverride and delete any files inside.