What version of Katello are you on?
We recently reworked the UI for Debian/Ubuntu repos to add more sanity checking, instructions, and UI hints (but I don’t think that change is released yet).
Not sure if that is the error you are running into, but in addition to Upstream URL, you should at least set the “Releases” field (now named “Releases/Distributions” with our latest changes). This is a required field for Debian/Ubuntu repos (the latest changes will throw a warning if you set “Upstream URL”, but leave “Releases/Distributions” blank).
@MSinghal Does the undefined method 'upstream_authentication_token_changed error sound familiar to you?