After uninstalling the native versions of nodejs and npm from debian 10, I installed nodejs 16.14.0 thus:

cd ~
curl -sL https://deb.nodesource.com/setup_16.x -o nodesource_setup.sh
sudo bash nodesource_setup.sh
sudo apt-get install -y nodejs

From the project directory I initialized the project and installed express.js and pug using a special option to avoid a stupid error:

npm init
npm install express --no-bin-links
npm install pug --no-bin-links