Jekyll 4.0 was released on August 20, 2019. However, GitHub pages does not natively support 4.0 today. They currently support only version 3.8.5 today. There are ways to pre-compile you application using Jekyll 4 and deploy the contents to the gh-pages branch of your GitHub pages application. One way you can accomplish this is by using a GitHub action such as the one built by Bryan Schuetz. GitHub also limits the dependencies they support out of the box. So, if you’d like to use jekyll-admin, for example, you’ll have to pre-compile. I recommend that before starting to use GitHub Pages with Jekyll, you are certain you are aligning your dependencies with those found on GitHub Pages. The site will help you with this.