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