npm is a virtual package provided by nodejs, it does not need to be installed.

docs/getting-started.rst

@@ -53,7 +53,7 @@ package manager. On Debian/Ubuntu-based distributions::
   curl https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo apt-key add -
   sudo sh -c 'echo "deb http://apt.postgresql.org/pub/repos/apt/ $(lsb_release -cs)-pgdg main" > /etc/apt/sources.list.d/pgdg.list'
   sudo apt update
-  sudo apt install python3 python3-venv libsvn-dev postgresql-10 nodejs npm \
+  sudo apt install python3 python3-venv libsvn-dev postgresql-10 nodejs \
                    libsystemd-dev
 
 Postgres