nvm is for installing node not npm, npm will be installed

What you're doing with nvm install npm, is attempting to install a node version called npm.

Do nvm install 14, let it run, check for errors, then once installed your have npm.

Generally speaking, loading arbitrary data from a URL into a root shell session is not a good idea and I wish people would stop peddling it as a solution for everything - "Please just run this script I'm sending you, and also while we're at it - I have a bridge you'd probably be interested in purchasing".

As an alternative, here's the "Ubuntu Way" of doing the same, where you can see how the system is being updated and know what repositories and what keys are added to your system configuration:

curl https://deb.nodesource.com/gpgkey/nodesource.gpg.key | sudo apt-key add -
sudo apt-add-repository "deb https://deb.nodesource.com/node_7.x $(lsb_release -sc) main"
sudo apt-get update
sudo apt-get install nodejs

And here's the "post deprecation of apt-key way" of doing the same thing:

curl https://deb.nodesource.com/gpgkey/nodesource.gpg.key | gpg --dearmor | sudo tee /etc/apt/keyrings/nodesource.gpg
echo "deb [signed-by=/etc/apt/keyrings/nodesource.gpg] https://deb.nodesource.com/node_7.x $(lsb_release -sc) main" | sudo tee /etc/apt/sources.list.d/nodejs.list
sudo apt update
sudo apt install nodejs

This is for the latest (at time of writing) Nodejs version 7. Other versions can also be gotten with a simple change to the repo URL - consult nodesource.com documentation for details.