If your GitHub Pages site includes a symbolic link (also known as a symlink) to another file or directory that does not exist within your site's repository, your site will not build.
If your GitHub Pages site fails to build because of an invalid symlink, you'll get an email that looks like this:
Subject: Page build failed
The page build failed with the following error:
The symbolic link `/foo` targets a file which does not exist within your site's repository.
Troubleshooting invalid symlink errors
If you do not intend the indicated file to be a symlink, or if you do not use the indicated file to build your site, in many cases, you may safely delete it.
If the symlinked file is necessary to build your site, make sure the file or directory it references is in your repository. To include external assets, consider using a git submodule or third-party package manager such as Bower.