user@pihole:~$ lsb_release -a
No LSB modules are available.
Distributor ID:	Debian
Description:	Debian GNU/Linux 9.4 (stretch)
Release:	9.4
Codename:	stretch

user@pihole:~$ uname -r
4.9.0-6-amd64

user@pihole:~$ systemctl --failed
  UNIT             LOAD   ACTIVE SUB    DESCRIPTION
● lighttpd.service loaded failed failed Lighttpd Daemon

LOAD   = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB    = The low-level unit activation state, values depend on unit type.

1 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'.