Installation and configuration of Splunk Enterprise is a prerequisite for the installation and configuration of Splunk IT Service Intelligence. Here is a link to the installation section of the Splunk IT Service Intelligence documentation manual:
Splunk Enterprise is the core platform on which Splunk ITSI App would be installed. Splunk ITSI is dependent on Splunk Enterprise. In fact, almost all the apps on Splunkbase require Splunk Enterprise to be pre-installed.