According to Github's own documentation: https://docs.github.com/pt/pages/getting-started-with-github-pages/creating-a-github-pages-site

Github will look for the index.html file

"Create the entry file for your site. GitHub Pages will look for an index.html, index.md, or README.md file as the entry file for your site."

From what I saw, your file is called 'registration form.html', change it to index.html, and whenever possible avoid spaces in file and folder names

I hope it works!