First, check that Node.js (v12 or later) and npm are installed:

node -v
npm -v dependencies

If they are not installed, you can use homebrew to install them:

brew install node

Next, install dependencies that are requiered to run the website locally using npm:

npm install

Finally, serve the site locally:

npx @11ty/eleventy --serve

The site will be at http://localhost:8080/.