On avatars, we usually only use the built-in/VRChat shaders for mobile variants.
The PC Variants use custom shaders like poiyomi and liltoon.
Meaning the Multibuild will always fail since we can't target other avatars in the scene for different platforms.
As is, Multibuild is only really useful for world creators.
Please add an option to assign different avatars for mobile builds, and also switch back to the original build target after completion.