I am currently trying to figure out authentication. I am drawing a huge blank right now and thought I would reach out for help. I am trying to get to the default sign in for Portal or ArcGIS but instead it pops up with this. Any help would be greatly appreciated.
That sign in dialog appears when an item is encountered and the access to it has failed to present valid authentication with an access token. However, you didn't provide the context within which this message appears so I am not clear what you are asking for.
If you are developing a custom app, you can review information about creating access tokens with code in the security and authentication guide on the developer's website https://developers.arcgis.com/documentation/security-and-authentication/get-started/
If you are logging a user in via your portal then review the documentation here https://developers.arcgis.com/documentation/security-and-authentication/user-authentication/how-to-i...