If you've just installed VBox and it is not starting or working, the most common problem is usually that you don't have your kernel source installed, which means there is no kernel driver for vbox so it can't work.
So the first thing you should do is install your kernel source by running this:
sudo apt-get install linux-headers-`uname -r`
Then install the dkms/kernel module for vbox
sudo apt-get install virtu........
If you get error messages like this it is usually because /var/run/sshd does not exist.
root@userbox:/# service sshd status
● ssh.service - OpenBSD Secure Shell server
Loaded: loaded (/lib/systemd/system/ssh.service; enabled; vendor preset: enab
Active: failed (Result: start-limit-hit) since Wed 2019-04-10 02:24:44 EDT; 1
Process: 511 ExecStartPre=/usr/sbin/sshd -t (code=exited, status=255)........
systemd is like the service manager for your Centos and other modern Linux distributions (including Debian/Mint/Ubuntu) allows you to enable services, stop them, restart them, check their status and even reboot your system.
The key commands or arguments you will use with systemctl are the following:
list-units [PATTERN...] List loaded units