Had the same issue. USDZConvert 0.63 is buggy, Download and install 0.64 over the existing install and it should start to work (as far as this error is gone for me on Mojave). See also https://developer.apple.com/forums/thread/131868?login=true&page=1#633813022

@manemanero I can confirm that simply load and install the 0.64 Version over an existing 0.63 install solves the mentioned failure in thgis topic in the pythonpath missing to find the pxr module. I am on Mojave.