Summary

Creating a Sample Plan in Advanced roadmaps fails if the Setting "Allow unassigned issues" is turned OFF

Diagnosis

The UI does not show the generic error "Try to create your Sample Plan again". 

From the internal logs, the below error log can be seen:

   level: ERROR
   message: com.atlassian.rm.jpo.env.issues.IssueRequiredFieldsMissingException {Required Fields not set. [assignee]}

Solution

If the Jira Application option "Allow unassigned issues" is turned OFF, it marks the Assignee field Required. 

This option needs to be turned ON so that the Assignee field becomes Optional and the Sample plan can be created.