Comment 26 for bug 1725779

I found this to work with openvpn via the network manager:

I don't know if the message is directly telling you this (it doesn't seem to), but it turns out that for some reason the Network Settings doesn't save your password. Look in /etc/NetworkManager/system-connections/ for a file that matches the name you gave your vpn in the settings. In my case it was just called 'client'. Open it as root in an editor.

sudo emacs /etc/NetworkManager/system-connections/client
Then change this line in the [vpn] section:

password-flags=1
to

password-flags=0
Then add this section.

[vpn-secrets]
password="your-password"
Where "your-password" is your vpn password. The quotation marks around your password are required.

From:

https://necromuralist.github.io/posts/openvpn-on-ubuntu-1804/