cat /etc/network/interfaces No such file or directory

Since version 18.04 Ubuntu by default uses netplan to configure network.

Go to /etc/netplan, here you will find your network configuration.

It will look something like this:

00-installer-config.yaml

# This is the network config written by 'subiquity'
network:
  ethernets:
    ens18:
      dhcp4: true
  version: 2