It looks like there is an issue enabling an application for XBOX Live Creators program. The screen times out and throws fatal javascript errors. I tried in IE 11 and Google Chrome. Waited 15+ minutes. It's just bugged. Here is a screenshot with the Javascript stack trace:
Thanks for reporting this Shaun - you have indeed found a bug on our side :)
The Xbox Live Creators Program should support any UWP, regardless of when you created it, but we need to fix up something on our back end. We are digging and will let you know as we have a plan.