Symptoms
When you try to open a shared Calendar folder in Microsoft Outlook 2016 for Mac or Outlook for Mac 2011, you receive the following error message even though you were granted permission to the folder:
Outlook cannot open the folder. You do not have permission to open this folder. Contact <Calendar owner's name> for permission.
Cause
This issue occurs because the owner of the folder that you want to access shares the subcalendar folder with you but does not share their default (primary) Calendar folder.
Resolution
To fix this issue, the calendar owner must share both the primary and the secondary Calendar folders with you.
More Information
In Outlook for Mac, you must have at least Reviewer permission to access the other user's default Calendar folder to be able to also access any subcalendar folders.
For example, you have permissions to the following shared Calendar folders:

To be able to open the other user's Second Calendar folder, you must have at least Reviewer permission to their default Calendar folder.
For example, you have permissions to the following shared Calendar folders:
- Calendar
- Second Calendar

To be able to open the other user's Second Calendar folder, you must have at least Reviewer permission to their default Calendar folder.
References
For more information about how to share a folder in Outlook for Mac, see the following Office website: