Unity packages must be imported in the following order: VRCSDK > Shaders > Materials&Textures Package > Clothes Package
The Unity importer sometimes bugs out, resulting in prefabs missing materials. To fix it:
Find the FBX, right click, reimport.
Find the prefab, right click, reimport.
If your project contains custom shader made from unity shader graph (Unity 2022), you must enable unity shader graph by:
Navigate to package manager.
Change the filter to Unity Registry.
Search “Shader Graph” and press “Install”.
You should be using Modular Avatar to wear clothes. Use this site to add it to your VCC.
Modular Avatar | Modular Avatar (nadena.dev)
Here is the documentation on how to wear clothes.
Configuring a simple outfit | Modular Avatar (nadena.dev)