It seems that Ubuntu 18 does not use anymore the /etc/networking/interfaces file and it uses netplan instead.
Are the lab 35.1 and the chapter 35 going to be updated?
please notice Lab 35.1 anticipates there can be changes in the files in /etc, and the box at the top of the lab says:
Exercise 35.1: Static Configuration of a Network Interface
You may have to use a different network interface name than eth0 . You can most easily do this exercise with nmtui or
your system’s graphical interface. We will present a command line solution, but beware details may not exactly fit your
distribution flavor or fashion.
We did not give a GUI solution since it would have involved a lot of screen shots and different among distros and versions and is pretty easy to work out anyway, as is nmtui. This is why we don't advocate crawling directly into /etc/ configuration files any more, for these kinds of basic operations.
As best I can see with a quick look, section 35.11 on NIC Configuration files is where the specific files are mentioned, and even there it says "Depending on your version, these might be.." and then it says at the end it is better to use Network Manager.
LFS201 had an update very recently (less than a month ago I believe) and it is not possible to do real-time minor updates -- the entire course has to be done at once. I'm sure we will work in netplan at the next time, but barring major problems, that won't
be for a while.