Here is instructions for setting up the Built-in VPN Client on macOS High Sierra.
Click on the plus sign
Choose L2TP over IPSec and choose a name for VPN connection and click Create
Add the Server address and user name and click Authentication Settings
Add the password for your account and the pre shared secret and click OK
Thick the box Show VPN… and click Apply.
The VPN is now configured. To start the VPN click on the new VPN status icon on top meny, that will open a meny so you can start the VPN.