Seems a new annoyance under Debian 10 is by default the /sbin and /usr/sbin paths are not part of the shell environment when you su to root. Workaround is to edit the /etc/login.defs and add "ALWAYS_SET_PATH yes" so the config should look something like this:
#
# *REQUIRED* The default PATH settings, for superuser and normal users.
#
# (they are minimal, add the rest in the shell startup files)
ENV_SUPATH PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
ENV_PATH PATH=/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games
ALWAYS_SET_PATH yes
Relevant details found at
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=918754