It looks like that I was trying to execute the wrong yarn, because simply running sudo apt install yarn on my Ubuntu 18.04 gave me yarn from cmdtest.

So I solved by uninstalling it:

sudo apt remove yarn

And by installing it as the official website explains, which in my case (Ubuntu 18.04) it was the following:

curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -
echo "deb https://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list

sudo apt update && sudo apt install yarn

yarn

You've got the wrong yarn. The yarn you're executing comes from the cmdtest package. Uninstalling cmdtest first should fix this:

sudo apt remove cmdtest

Once you've uninstalled it, run the commands below to install yarn properly:

curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -
echo "deb https://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list
sudo apt update
sudo apt install yarn

The real name you should use when installing is yarnpkg

sudo apt install yarnpkg

That is the solution.