This specific item, often part of the "Phoebe & Vanessa" collection, typically features a stylish, high-quality dress designed for feminine Sims. These files are commonly hosted on platforms like Patreon or The Sims Resource.

In the game's Game Options > Other menu, check "Enable Custom Content and Mods".

If your Sim looks "chopped up" or has red/white question marks, you might be missing the mesh file. Check the creator's page to see if you need to download a "required" base file first.