The prototype kit is still in development and is not ready to use.
Install the long-term support (LTS) version of Node.js, which includes NPM.
Clone the repo: git clone https://github.com/nhsuk/nhsuk-prototype-kit nhsuk-prototype-kit and whilst in the project directory cd nhsuk-prototype-kit, install the required npm packages with: npm install
Run the project in development mode npm run watch and visit http://localhost:3000.