How to start openvpn or ssh server under Ubuntu 16.04 and 18.04

sudo systemctl start openvpn@client
The "client" is derived from the configuration file name (/etc/openvpn/client.conf). If your configuration file is server.conf, the command needs to be
sudo systemctl start openvpn@server
When you want the service to start up automatically during system boot, you issue:
sudo systemctl enable openvpn@server
Templated versus non-templated services
OpenVPN is a so-called "templated" service (it needs a configuration file when being invoked). In contrast to this, the ssh server is a non-templated service. Hence the command is simpler:
sudo systemctl start sshd