Just to complement Nic's answer, Service Desk 2.x is only compatible with JIRA 6.3.5+ as described on the URL below. In case you're using a lower version you'll need to upgrade JIRA to at least 6.3.5 version.
...be more productive while being fun to use at the same time. For some, getting started can be a bit intimidating. This is especially true if Jira Service Desk is your first exposure to Atlassian...
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