Apple - How can I read the log when I fail with VPN

Whether you choose L2TP over IPSec or PPTP for your settings, the log file is located at:

/var/log/ppp.log

If you want use terminal to view your log file you can do following:

vim /var/log/ppp.log

tail -f /var/log/ppp.log (if you want see end of file)

less /var/log/ppp.log (if your log file was huge and want to see page by page)

For completeness, I'll add a way to view the log in the GUI.

Open the Console utility, select Show Log List from the toolbar (if it isn't already shown), then select FILES > /private/var/log > ppp.log from the sidebar.

On macOS Catalina, the toolbar reads "Log Reports", then ppp.log