HI,
I'm trying to follow the tutorial from the beginning. In my desktop PC -running Ubuntu 18.04- I open a terminal and:
mkdir devel cd ~/devel
git clone https://github.com/OctoPrint/OctoPrint [...] cd OctoPrint
$ virtualenv venv
Oops! virtualenv command is not installed, so I run:
$ sudo apt install python3-virtualenv
but nothing changes.
I'm very confused
try sudo python3 -m pip install virtualenv or pip install virtualenv?
I've just tryed with
sudo apt-get install python-virtualenv virtualenv
It seems that works OK.
Make sure that's using Python 3 and not Python 2. I might take a look at the docs.octoprint.org instructions and make sure they're not out of date, since it is easier to use python3 -m venv venv - virtualenv, by default will always use the Python 2 install 
You are right. Running
$ virtualenv venv
I get "Running virtualenv with interpreter /usr/bin/python2..."
so I run
$ python3 -m venv venv
and now octoprint serve reports:
[...]
INFO - Detected environment is Python 3.6.9 under Linux (linux).
[...]
it would be nice to include a note on the Tutorial page, warning Ubuntu users of those changes
Great that you got it up and running. All the install guides are up-to-date, but some of the stuff on the documentation is less looked at. Will add it to my list of things to do on the docs 
It's not just Ubuntu users that will be caught out by this - Python 2 is being dropped by many distros now, for example in Debian 11 (bullseye) & in Ubuntu 20.04 I don't think Py2 is even available through apt anymore. Can definitely get confusing, will switch it over to using python -m venv asap.
Thank you for your work.
For my part, I will continue studying the tutorial. 