Install fdf2xsf.
Or if it's installed, ensure it's in a directory which is in $PATH.
Or when referencing it, use the full path to fdfsxsf. E.g.
Code:
{/usr/local/bin/fdf2xsf < ch.fdf > ch.xsf}
What is fdf2xsf? Typing 'fdf2xsf' in to Google currently gets only one hit, which is your post.