Laundry Day Stuff Pack Not Showing

Simone61298  If you bought the pack through Steam, then yes, you need to install Sims 4 itself through Steam in order to use it.  Make sure the EA App version of Sims 4 is already uninstalled.  You could then install any EA App-owned packs through the App, and everything would load as if you owned it all through Steam.  The opposite doesn't work though, which is why you need to play through Steam.

If you open the EA App and click Sims 4 > Manage > Manage add-ons, you should see Laundry Day listed with a "Manage on Steam" tag.  That would be true for any pack you bought through Steam.  In order to NOT use Steam and still have access to this pack, you'd need to unlink the Steam and EA accounts.  If you don't want to do that, you might be able to remove the pack from your Steam library like this:

https://answers.ea.com/t5/Technical-Issues-PC/Free-Pets-stuff-no-install/m-p/13240592#M263979

If that doesn't work, you could try to get Steam or EA customer support to remove the pack.  Make it clear you don't want a refund, just to have the content removed.

The point here is that as long as the EA App sees a Steam entitlement for Laundry Day, it's likely going to refuse to install the pack and direct you to Steam instead.  You could of course try buying the pack on ea.com to see whether it helps; if you're going to purchase it again anyway, it doesn't matter whether you do it before or after.  But it's very possible that the Steam entitlement will override anything else as long as it's present.