Don't do this unless you are a Jekyll developer. You don't need to install Jekyll on your Mac to test your website locally. Use Docker instead. There is a learning curve, but you can use your Docker skills for much more than just testing a static website.
Briefly, you need to (1) download and install Docker Community Edition for Mac, (2) create a DockerHub account and download a Docker image that's configured to run the GitHub Pages software, and (3) learn how to create a container and communicate with it. This is all documented online. Contact me if you need help.
Briefly, you need to (1) download and install Docker Community Edition for Mac, (2) create a DockerHub account and download a Docker image that's configured to run the GitHub Pages software, and (3) learn how to create a container and communicate with it. This is all documented online. Contact me if you need help.