Htmlfontify has a couple of external requirements:
Other versions may work—these have been used successfully by the author. If you intend to use Htmlfontify in batch mode, 21.1+ is pretty much required. The author does not know if XEmacs, NTemacs, or J.Random Emacs will run Htmlfontify, but reports/patches/bags of money are always welcome.
You may be able to work around this with a suitable clever shell command and the customization entry: hfy-find-cmd