Check your Github setup. They might (I know if from Netlify only) have an option to “block all bots” in their security setup for your pages. In that case the Google Bot is blocked. Also check if your robots.txt is maybe blocking Google.
Edit: Cloudflare is the one with the Bot blocking.
The site is in /RUB/ and the links in the sitemap are in /RUB/ so that is ok. A sitemap has to be at least in the lowest directory of the links it contains, so this is ok.
A quick Google search brought some issues with Github hosted sites and webmaster tool access. Seems to be a recurring issue. I wonder how you “validated” your site with Google Webmaster tools. If you can validate it by adding a file to the site, we know that it’s a problem with your sitemap, if you can’t validate (I would expect that result) then we know that Google can’t access your Github pages. The docs on Github don’t say anything about blocking bots… not sure if that is a configurable thing.
Do you use Cloudflare on top of that? Switch off the bot-blocker at Security > Bots > Standard Bot Protection Settings.
Another, last, thought: looking for your site via site:https://nachiketa3299.github.io/RUB/ I can find only one single page, the home page. Are you sure you don’t have a penalty?