Recently, I have made an application on developer.spotify.com
I have added one account into "Users and Access" page, since I've remembered what the Full Name of the account was. Unfortunately, on the other hand, I have another account which's Full Name is not known to me.
I tried to add email + my display name and email + username (long string from settings). None of them worked. I still get User not registered in the Developer Dashboard, reason: None error from Spotipy.