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 https://pages.github.com/versions/ will help you with this.