E: Unable to locate package npm

From the official Node.js documentation:

A Node.js package is also available in the official repo for Debian Sid (unstable), Jessie (testing) and Wheezy (wheezy-backports) as "nodejs". It only installs a nodejs binary.

So, if you only type sudo apt-get install nodejs , it does not install other goodies such as npm.

You need to type:

curl -sL https://deb.nodesource.com/setup_14.x | sudo -E bash -
sudo apt-get install -y nodejs

Optional: install build tools

To compile and install native add-ons from npm you may also need to install build tools:

sudo apt-get install -y build-essential

More info: Docs NodeJs

Encountered this in Ubuntu for Windows, try running first

sudo apt-get update
sudo apt-get upgrade

then

sudo apt-get install npm

I ran into the same issue on Debian 9.2, this is what I did to overcome it.

Installation

sudo apt install curl
curl -sL https://deb.nodesource.com/setup_6.x | sudo bash -
sudo apt-get install -y nodejs
sudo apt-get install -y npm

Check installed versions

node --version
npm --version

Originally sourced from "How to install Node.js LTS on Debian 9 stretch" http://linuxbsdos.com/2017/06/26/how-to-install-node-js-lts-on-debian-9-stretch/