Make sure that you have enabled L2TP/IPsec. This can easily be done via the SoftEther VPN Manager.
If L2TP/IPsec is in fact enabled, but you can’t connect, it could be that the client can’t get an IP address from the server. You can either use SecureNAT or set up a DHCP server to do this. We’ll go with the first option, since it’s the easiest.
If you’re still unable to connect, it is most likely your firewall on the server causing the issue. You need to have the following ports open/accepting connections below:
On an Ubuntu server, this is really easy. You can just use UFW.
sudo ufw allow 500,1701,4500/udp
If you’re using SSH, you’ll probably want to allow SSH with
sudo ufw allow ssh
Don’t forget to enable the firewall with
sudo ufw enable!
If all else fails, let’s make sure that you’ve set up the L2TP client correctly. Here’s what it’ll look like on Windows, and it should look similar on other devices.