Your screenshot literally gives you the answer: it's not available for your instance. 

Your 'details' give every possible instance except 'demo'. So on what kind of instance are you trying to install it. Also: if you don't have ITSM, how do you want to integrate it with Incident mgt? What is installed on the instance? 
Checking the Store: it's a free app for Zurich, Yokohama and Xanadu instances that either have ITSM or ITOM installed.