install yarn on Ubuntu16.04LTS

# configure the repository
$ 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

# On Ubuntu 16.04 or below and Debian Stable, 
# you will also need to configure to get a new enough version of Node.js.
# install Node.js 10
$ curl -sL https://deb.nodesource.com/setup_10.x | sudo -E bash -
$ sudo apt-get install -y nodejs

# To compile and install native addons from npm 
# you may also need to install build tools
$ sudo apt-get install -y build-essential

# install it
$ sudo apt-get update && sudo apt-get install yarn
# check version
yarn --version

reference:
1. Debian / Ubuntu
2. Debian and Ubuntu based Linux distributions