Right after Calendar's installation first you need to do is go to Calendar's administrator menu and set 2 custom fields of Date or Date Time types for Start Date and End Date. The fields can be created or you can use automatically generated field by app.
Calendar's administration can be reached by going to JIRA administrator menu Jira settings / Apps/ Teamlead / Calendar.
Add Start Date / End Date custom fields to Create and Edit Issue screens, otherwise the issues will not be visible in the calendar.