The su command in buster is provided by the util-linux source package,
instead of the shadow source package, and no longer alters the PATH
variable by default. This means that after doing su, your PATH may not
contain directories like /sbin, and many system administration
commands will fail. There are several workarounds:

Use su - instead; this launches a login shell, which forces PATH to be
changed, but also changes everything else including the working
directory.

Use sudo instead. sudo still runs commands with an altered PATH variable.

To get a regular root shell with the correct PATH, you may use sudo -s.

To get a login shell as root (equivalent to su -), you may use sudo -i.

Put ALWAYS_SET_PATH yes in /etc/login.defs to get an approximation of
the old behavior. This is documented in su(1) but not in
login.defs(5). It may also cause a harmless error message to appear in
some situations (see 905564).

Put the system administration directories (/sbin, /usr/sbin, /usr/local/sbin) in your regular account's PATH (see EnvironmentVariables for help with this).

https://wiki.debian.org/NewInBuster#Changes

https://manpages.debian.org/buster/util-linux/su.1.en.html

login: shows error about "ALWAYS_SET_PATH" which is supported by "su" from "util-linux"
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=905564