I got this error and now I can’t update my avatar. It may have to do with the fact that I tried to use an Upload to Android plug-in which I later deleted, but now I can’t upload to PC either.
Most likely based on what is suggested, it’s a module you have to download from the unity web page so your unity editor can build the project.
But i suppose that this module should be installed by default. Are you running this on another platform like Linux or MacOS ?