Cause 1: make sure you have an index.html file

Cause 2: make sure that in GitHub Pages (under settings) your source is the
master branch

For anything else look here: https://stackoverflow.com/questions/45362628/github-pages-site-not-detecting-index-html#:~:text=Go%20to%20settings%20for%20your,the%20gh%2Dpages%20branch'.