The most common reason for this error is that a user is requesting their username and password, but a username has not been set up in the system for them yet.

To view/add/edit a user's username, a Platform Manager will need to go to:
  1. Core > People Finder > Search/select your user
  2. Select the Access tab
  3. Under Log In Information, you can view/edit or add a username here

Alternatively, you can also access this through Edit User Profile Data.
  1. As a Platform Manager, navigate to 
  2. Core > Users > Edit User Profile Data
  3. Search for and select the User
  4. Scroll down to Sign In Information
  5. Click Edit to add/edit a username
  6. Click Save