Making 'Assignee' field required when 'Allow unassigned issues' is turned ON in General Configuration
Diagnosis
The Assignee field is required or automatically assigned by default when Allow unassigned issues is turned OFF in Administration > General Configuration.
If it is set as ON, user will be allowed to choose the Unassigned option and this affect every project in JIRA. In certain use case, some project might need the Assignee field to be required and "unassign" is not an option. This feature has already been requested in JRA-1001 - Getting issue details... STATUS
Workaround
There is no way to do it by default in JIRA. However, this can be achieved by using a third party plugin called JIRA Suite Utilities, which has a set of conditions and validations to personalize workflow of JIRA.
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- Install the JIRA Suite Utilities. For install the plugin please refer to: Installing JIRA plugin
- Use the 'Fields Required' validator function in your specified transition. Please refer to: https://studio.plugins.atlassian.com/wiki/display/JSUTIL/JIRA+Suite+Utilities+Workflow+Validators
- Add the 'Assignee' field as required.