It looks like someone was getting that error and there was a discussion on Slack about it in #tech-questions. The below message seemed to have helped:
"The redirect URL would be used when sending the user back to your app after account linking. Example. I am using your app. You send me to the account linking screen. After I enter in my credentials, you would be redirected back to your app or web. Localhost is used when there is no redirect URL"