My guess is that deeper in the build process there is a default value for UBTPath, the default install location. Wherever that is, that is what is creating the Error_UBTMissing error message which coincidentally (perhaps intentionally?) mirrors the name of the goto in Clean.bat.
Anyway, easy fix here, hopefully someone commits this before the next preview release, as currently it is 100% broken for non-default location installs, at the very least.