@Jazeeriii Some custom content gets classified as store content, for whatever reason, so please go through the categories again and make sure the dress is in fact not installed. If it's not, try repairing the game: open your Origin game library, right-click on the Sims 3 icon, and select Repair.
If that doesn't help, please move the DCCache folder, inside the Sims 3 folder, to your desktop, then install the dress again and see whether it shows up. Moving this folder will effectively uninstall your store and custom content, but that can be addressed laster if you don't want to reinstall everything.
I feel like this is the cause of the problem. I tried uninstalling the game and installing it again and this pops up, with this showing I can't repair The Sims 3 but the game is playable.
@Jazeeriii Try running the Origin reset tool, then see whether you can repair.
https://help.ea.com/en-us/help/origin/origin/how-to-reset-origin-on-mac/
