Comment # 10 on bug 1182835 from Fabian Vogt

pam_systemd will not create sessions if you're already in a session. You can
only use systemctl --user from within sessions such as local logins on the tty
or X11, or remote logins using ssh.

See also https://github.com/systemd/systemd/issues/7451#issuecomment-346787237.