Currently, the following message is displayed when it is not possible to create an issue through the Service Desk:

Not able to create issue via this Service Desk, please contact the system administrator.

In order to see what is the problem, it is needed to check the logs, which will show errors similar to:

2013-10-08 10:49:46,822 XXXXXXX ERROR sysadmin 649x10928x1 mibv2 XX.XXX.XXX.XXX,XXX.XXX.XX.XXX /servicedesk/customer/ssd/create/get-it-help [atlassian.viewport.fields.FieldValidator]  [assignee] The default assignee does NOT have ASSIGNABLE permission OR Unassigned issues are turned off.

It'd be good and avoid friction if the reason it is not possible to create the issue is shown to the Administrator. That is, if an Administrator tries to replicate the problem, a message as the following would be displayed in the screen:

Not able to create issue via this Service Desk, the default assignee does NOT have ASSIGNABLE permission OR Unassigned issues are turned off.

Knowledge Base Article: Not able to create issue via this Service Desk error