No internet connection after ubuntu server 20.04 install, ifconfig not available

A short addon to chili555's answer: In my case, the only problem was that for some strange reason, Ubuntu believed my network interface name in /etc/netplan/01-netcfg.yaml to be enp0s5 instead of enp0s4 which was the correct for my VM.

Networking in Ubuntu server is managed by netplan. Your file lacks the details needed to connect the ethernet properly.

First, let's rename the file:

sudo mv /etc/netplan/*.yaml  /etc/netplan/01-netcfg.yaml

Now, let's change it to include the required details:

sudo nano /etc/netplan/01-netcfg.yaml   

Change the file to read:

network:
  version: 2
  renderer: networkd
  ethernets:
    ens33:
      dhcp4: true

Netplan is very particular about spacing, indentation, etc. Proofread carefully twice. Save (Ctrl+o followed by Enter) and exit (Ctrl+x) the text editor. Follow with:

sudo netplan generate
sudo netplan apply

You should be all set.