You are referring to the section “2.3.5 Deploying the demo app” of this “Ruby on Rails Tutorial “:
In section 2.3.1 Planning the application, note that they did:
$ git remote add origin git@github.com:<username>/demo_app.git
$ git push -u origin master
That is why a simple git push worked (using here an ssh address).
Did you follow that step and made that first push?
www.github.com/levelone/demo_app
That would not be a writable URI for pushing to a GitHub repo.
https://levelone@github.com/levelone/demo_app.git
This should be more appropriate.
Check what git remote -v returns, and if you need to replace the remote address, as described in GitHub help page, use git remote --set-url.
git remote set-url origin https://levelone@github.com/levelone/demo_app.git
# or
git remote set-url origin git@github.com:levelone/demo_app.git