Why can't profiles set to External User login to the console?
Oracle Service Cloud, Service Collaboration
August 2014 and newer versions
A profile set to External User is allowed collaboration-only access, and cannot log in to the console. To check for this, click the Other tab and examine the External User check box. Service Collaboration must be enabled for this check box to appear.
For additional information, refer to the 'Collaborating with external users' and/or 'Customizing profiles' section in online documentation for the version your site is currently on. To access Oracle Service Cloud manuals and documentation online, refer to the Documentation for Oracle Service Cloud Products.