L2TP VPN is not working on Ubuntu 20.04. Same credential is working fine with Ubuntu 18.04
The package network-manager-l2tp is required.
You may have to install it:
sudo apt install network-manager-l2tp
I hope it helps
I was facing this myself in Ubuntu 20.04 and 21.04, and a simple one-liner solved all requirements necessary for my L2TP/IPsec setup to work properly:
sudo apt install --install-suggests network-manager-l2tp-gnome
This installs all packages mentioned in the previous answers, too. Endpoint to connect to is a Ubiquiti EdgeRouter ERLite-3.