1. Move the .ovpn file to /etc/openvpn/. Choose the <name-of-connection> carefully, the name is required to select the connection via systemd.

   mv <config-file>.ovpn /etc/openvpn/<name-of-connection>.conf

2. Reload the daemon:

   systemctl daemon-reload

3. Start the connection:

   systemctl start openvpn@<name-of-connection>

4. Verify that everything worked correctly:

   systemctl status openvpn@<name-of-connection>

   If it says running and no errors are displayed, everything was successful. If it says cipher not allowed check the following paragraph.

5. Enable it with:

   systemctl enable openvpn@<name-of-connection>

6. To later disable the automatic start of the connection use:

   systemctl disable openvpn@<name-of-connection>

Sourced from smarthomebeginner.com - OpenVPN autostart

1. If there are the following lines in the console output of the connection:

   Error: negotiated cipher not allowed - AES-256-CBC not in AES-256-GCM:AES-128-GCM:CHACHA20-POLY1305
   OPTIONS ERROR: failed to import crypto options
   Failed to open tun/tap interface

2. Edit your config file. (Maybe under /etc/openvpn/) It's required to change the line that contains cipher. Edit cipher to data-ciphers. It should then look like this:

   data-ciphers AES-256-CBC

3. Finally restart your connection.

Partially sourced from forums.openvpn.net - Cipher not allowed