cant start systemd - Failed to get D-Bus connection

Resolution

Install systemd-sysv:

apt install systemd-sysv

Explanation

Failed to get D-Bus connection: Unknown error -1 means that systemd is not running.

Here is my LMDE 2 installation, which is a lot like Debian 8:

lmde ~ # systemctl status ssh.service
Failed to get D-Bus connection: Unknown error -1

On your system, /sbin/init probably isn't a symlink pointing to /lib/systemd/systemd. Here's what it looks like on LMDE 2:

lmde ~ # file /sbin/init
/sbin/init: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=416c01d5df07b3a795694ff3360339b509daaf86, stripped

Here's what it looks like on Ubuntu 16.04 LTS, which runs systemd by default:

root@node51 [~]# file /sbin/init
/sbin/init: symbolic link to /lib/systemd/systemd

To resolve this, replace the package sysvinit-core with systemd-sysv:

lmde ~ # apt install systemd-sysv
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be REMOVED:
  sysvinit-core
The following NEW packages will be installed:
  systemd-sysv
0 upgraded, 1 newly installed, 1 to remove and 533 not upgraded.
Need to get 32.7 kB of archives.
After this operation, 177 kB disk space will be freed.
Do you want to continue? [Y/n] Y
Get:1 http://packages.linuxmint.com/ betsy/upstream systemd-sysv amd64 215+12+betsy [32.7 kB]
Fetched 32.7 kB in 0s (81.1 kB/s)
dpkg: sysvinit-core: dependency problems, but removing anyway as you requested:
 init depends on systemd-sysv | sysvinit-core | upstart; however:
  Package systemd-sysv is not installed.
  Package sysvinit-core is to be removed.
  Package upstart is not installed.

(Reading database ... 159705 files and directories currently installed.)
Removing sysvinit-core (2.88dsf-58) ...
Processing triggers for man-db (2.7.0.2-5) ...
Selecting previously unselected package systemd-sysv.
(Reading database ... 159681 files and directories currently installed.)
Preparing to unpack .../systemd-sysv_215+12+betsy_amd64.deb ...
Unpacking systemd-sysv (215+12+betsy) ...
Processing triggers for man-db (2.7.0.2-5) ...
Setting up systemd-sysv (215+12+betsy) ...

Now reboot with reboot.

Once the system has rebooted, you'll be running systemd instead of SysV:

lmde ~ # file /sbin/init
/sbin/init: symbolic link to /lib/systemd/systemd

lmde ~ # systemctl status ssh.service
● ssh.service - OpenBSD Secure Shell server
   Loaded: loaded (/lib/systemd/system/ssh.service; enabled)
   Active: active (running) since Wed 2017-02-08 06:58:27 CST; 1min 10s ago
 Main PID: 598 (sshd)
   CGroup: /system.slice/ssh.service
           └─598 /usr/sbin/sshd -D

Feb 08 06:58:27 lmde sshd[598]: Server listening on 0.0.0.0 port 22.
Feb 08 06:58:27 lmde sshd[598]: Server listening on :: port 22.
Feb 08 06:58:29 lmde sshd[598]: Received SIGHUP; restarting.
Feb 08 06:58:29 lmde sshd[598]: Server listening on 0.0.0.0 port 22.
Feb 08 06:58:29 lmde sshd[598]: Server listening on :: port 22.
Feb 08 06:58:44 lmde sshd[1614]: Accepted password for root from 10.0.0.2 port 59703 ssh2
Feb 08 06:58:44 lmde sshd[1614]: pam_unix(sshd:session): session opened for user root by (uid=0)

Possible Cause?

You indicated that systemd on your Debian installation worked in the past. It is possible that a package or its dependencies required SysVinit instead of systemd and erased or disabled systemd.

Be on the lookout for broken dependencies if this is the case.

Additional Resources

Documentation

• systemd - Debian Wiki

Description of sysvinit-core

lmde ~ # apt show sysvinit-core
Package: sysvinit-core
State: not installed
Version: 2.88dsf-59
Priority: extra
Section: admin
Maintainer: Debian sysvinit maintainers <[email protected]>
Architecture: amd64
Uncompressed Size: 199 k
Depends: initscripts (>= 2.88dsf-13.3), sysv-rc | file-rc, sysvinit-utils (>=
         2.86.ds1-66), libc6 (>= 2.15), libselinux1 (>= 1.32), libsepol1 (>=
         1.14), debconf (>= 0.5) | debconf-2.0, debianutils (>= 4)
Conflicts: systemd-sysv, systemd-sysv, upstart, upstart, sysvinit-core
Replaces: systemd-sysv, systemd-sysv, sysvinit (< 2.88dsf-44~), sysvinit (<
          2.88dsf-44~), upstart, upstart
Description: System-V-like init utilities
 This package contains programs required for booting a Debian system and doing
 basic process management.

 The most important program in the package is /sbin/init. It is the first
 process started on boot and continues to run as process number 1 until the
 system halts. All other processes are descended from it.
Homepage: http://savannah.nongnu.org/projects/sysvinit

Description of systemd-sysv

lmde ~ # apt show systemd-sysv
Package: systemd-sysv
State: installed
Automatically installed: no
Version: 215+12+betsy
Priority: extra
Section: admin
Maintainer: Debian systemd Maintainers <[email protected]>
Architecture: amd64
Uncompressed Size: 77.8 k
Depends: systemd (= 215+12+betsy)
PreDepends: systemd
Conflicts: sysvinit-core, sysvinit-core, upstart, upstart, systemd-sysv
Replaces: sysvinit (< 2.88dsf-44~), sysvinit (< 2.88dsf-44~), sysvinit-core,
          sysvinit-core, upstart, upstart
Description: system and service manager - SysV links
 systemd is a replacement for sysvinit.  It is dependency-based and able to read
 the LSB init script headers in addition to parsing rcN.d links as hints.

 It also provides process supervision using cgroups and the ability to not only
 depend on other init script being started, but also availability of a given
 mount point or dbus service.

 This package provides the manual pages and links needed for systemd to replace
 sysvinit. Installing systemd-sysv will overwrite /sbin/init with a link to
 systemd.
Homepage: http://www.freedesktop.org/wiki/Software/systemd