Can Amazon Echo OAuth take an authorization url that is a registered domain name?

I am trying to set up OAuth with the Amazon Echo. Unfortunately, I get the error,

Error: A URL must be between 9 and 2000 characters.

When I try to set up OAuth/account linking on the Amazon Developer portal. It seems to be an issue with my authorization url I am giving Amazon.

I substituted out the actual url for privacy.

When I visit the url (a web page that is served through a Flask application), everything works fine. My thought is that since the url is not a registered domain name, maybe Amazon won't let me use that url.

When I looked at Amazon's documentation their example URL has a similar length to mine which is why I think the error may be a bit misleading.

What could be causing this error and how can I go about fixing it? Thank you :)

I believe it is failing since your page is not served via HTTPS. The documentation states, along with other items, 'It must be served over HTTPS.'