this article describes how Mixamo rigged Characters can be Imported into Unity to work with VRchat.
It is based on this amazing Blogpost:

The Unity Add-In is supposed to block you from uploading imporperly rigged Characters.
If however you fixed the Prefab after importing it, you can end up with your character not beeing animated.

How to change the Bone Structure

  1. Load the rigged Model into the Assets, set Rig to Humanoid and edit the bonestructure
  2. Then set the “Upper Chest” to None, “Chest” to Spine2, Apply and Done.
    Now you can import the Character and continue with the tutorial.