If you are having trouble deleting appointments on a shared calendar (including resource calendars) in Outlook for Windows and you see the error message below.
Exit all clients (such as Outlook and Outlook Web Access) on which the mailbox is set up, start Outlook, and then try to remove the item.
Log into Outlook Web Access (OWA – https://mail.utoronto.ca) to delete the appointment.
Additional methods from Microsoft