I have followed this guide to setup my GitHub Pages. My username is FReina, and therefore I thought that my website would be at FReina.github.io. However, I consistently get a 404.
Anybody know if this is due to the uppercase in my username?
The issue is simply caused by the fact that there is latency between your username.github.io repository on Github and the actual hosted website. Usually this is only a few minutes, but I have known it to be much longer than that when the site is busy.
From my PC, it looks like your site is now up, and displaying a Hello World page. If you still cannot see it, it may be that the 404 page is cached by your browser - try emptying your browser cache (on Chromium-based browsers including Google Chrome and newer versions of MS Edge, this is done by opening the Inspect Element tool, clicking-and-holding the reload button and selecting "Empty cache and hard-refresh")