I also encountered the same problem today (05/28/2020). Suppose that you have done everything right (instructions in https://pages.github.com/), you should have a repo named username.github.io and index.html set up.
What worked for me was that I chose a Jekyll theme. First, go to Settings of the repo. In GitHub Pages section, look for Theme Chooser then click on Choose a Theme. It will redirect you to a GitHub page that has multiple themes that you can choose from. Choose a theme that you like then click Select Theme. After doing these steps, I refreshed my username.github.io and the page worked correctly.
