Adding this because it was frustrating for me and I could not remember this solution when setting up a new device.
Firing up a valid VPN connection from these instructions (mostly) gave me the message:
"VPN connection XXXX failed because there are no vpn secrets."
This setup uses a USERNAME / PASSWORD for authentication.
Changing username/pass to remove ALL non-alphanumeric characters from the BOTH the username and password fixed this.
This requires making changes with your provider credentials
ex: my_username => my123username
I'm pretty sure this is an encoding issue, but I'm not sure where to bug-post it.