If the game crashes, use a tool like the Mod Squad's Sims 4 Tray Importer or a Mod Checker to find and remove conflicting content.
Relaunch the game for the dress to appear in Create A Sim (CAS) . Troubleshooting Tips
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.
Move the file into your Electronic Arts > The Sims 4 > Mods folder.