I just found this:
aborrero@tools-sgebastion-06:~$ sudo systemctl status user-.slice
● user-.slice
Loaded: error (Reason: Invalid argument)
Drop-In: /etc/systemd/system/user-.slice.d
└─puppet-override.conf
Active: inactive (dead)
Feb 04 11:14:26 tools-sgebastion-06 systemd[1]: [/etc/systemd/system/user-.slice.d/puppet-override.conf:11] Memory limit '0' out of range. Ignoring.
Feb 04 11:14:26 tools-sgebastion-06 systemd[1]: [/etc/systemd/system/user-.slice.d/puppet-override.conf:14] Unknown lvalue 'IPAccounting' in section 'Slice'
Feb 04 11:14:26 tools-sgebastion-06 systemd[1]: user-.slice: Slice name user-.slice is not valid. Refusing.
aborrero@tools-sgebastion-06:~$ apt-cache policy systemd
systemd:
Installed: 232-25+deb9u8
Candidate: 232-25+deb9u8
Version table:
239-12~bpo9+1 100
100 http://mirrors.wikimedia.org/debian stretch-backports/main amd64 Packages
*** 232-25+deb9u8 500
500 http://security.debian.org stretch/updates/main amd64 Packages
100 /var/lib/dpkg/status
232-25+deb9u6 500
500 http://deb.debian.org/debian stretch/main amd64 Packages- We have the wrong systemd version installed
- We have some typo in the config