I was able to run it installing npm and then installing sass package of npm.
I needed also to add a workaround to Error: CERT_UNTRUSTED
error, using the config set registry command.
before_install: - sudo apt update - sudo apt install npm - sudo npm config set registry http://registry.npmjs.org/ - sudo npm install -g sass
sudo gem install sass --version "=3.5.6"
Not working.
sudo apt install ruby-sass
Not working.