I am trying to build a page from a github repo in an org. Jekyll version 3.4.1
created repo - username.orgcustomdomain
If I add more info to index.md, it shows but it should have built _sites and that should have been displayed like it does locally.
That's your problem right there. It appears you're trying to setup your custom domain in an unsupported way. Here is official documentation to help you clear it up.
If that was wrong then I should be getting 404 right?
I tried adding data to index.md and that indeed works with my current configuration. But the auto generated site which goes to _site is not showing up.
And you're aware that Github Pages doesn't use _site right? That's just for local development. Pages instead attempts to generate it's own site and put that up. If you're using gems or plugins not included in the Github Pages Bundle then I'd recommend copying the contents of your local _site folder to your github pages repo and including a .nojekyll file in the root directory.
Also it looks like I am getting a 404 error.
Are you sure your repo is named bsreera.github.io?