CiviCRM 5.4, Unable to create case type (angular validation error)
This is due to the change made in this PR - https://github.com/civicrm/civicrm-core/pull/11998. 

I raised this on Mattermost some time ago. See https://chat.civicrm.org/civicrm/pl/d58c9nuowbfemeneizbe57cxqy. It says -


  Seems default assignee is a required field on Case Type Form(as per this change). Was this intentional? I was unable to save https://dmaster.demo.civicrm.org/civicrm/a/#/caseType/1 until I filled out default assignees for all the activities listed under standard timeline ‍♂️ 


But unfortunately haven't got any reply for the same.

To fix this  -


Remove the required word from this line of the PR and the following element. You should be able to save case type without selecting any value in the default assignee.


OR


Add values to the default assignee for all the activities.