Admins must assign the CalendarRulesTriggers permission set to users who need to use CalendarRules.

How to Assign the CalendarRules Permission set

1. Log in to Litify.
2. Navigate to Setup > Permission Sets > Litify Calendar Rules User.
3. Select Manage Assignments. 
4. Select Add Assignments. 
5. Enable the checkbox next to the user(s) that need the permission set.
   
6. Select Assign.