Cause of this error:

You are on a Windows based computer which has a registry entry for DelegateWastebasketStyle - see Microsoft 365 (Outlook for Windows/MacOS) - Sent or Deleted item is not saved within the sent/deleted items folder of the account actioned from article for more details. This registry entry is attempting to place the deleted item within the "Deleted Items" folder of the account you are deleting the event from - but you do not have the required permissions to this folder.

Workarounds:

Important: The workarounds are listed from best to worst. If the solutions listed below are not clear or you need further information before making a decision, please contact the DoIT Help Desk.

• Use Outlook on the Web for deleting calendar entries for this specific calendar. This is the simplest workaround with no side effects. However, this workaround might not be ideal if you only use Outlook for desktop.
  
• Delete the registry entry causing this issue. If you are linked to service account(s), when you delete an item (emails/calendar events), the deleted item goes into the "Deleted Items" folder of your account instead of the linked service account. Make sure you understand the ramifications of this change before proceeding. Follow instructions to delete the DelegateWastebasketStyle registry entry.
• Ask the calendar owner to assign you create permissions to their "Deleted Items" folder. See Microsoft 365 (Outlook on the web | Outlook for Windows/Mac) - Manage permissions to an email folder article - have them use Outlook on the web steps to assign these permissions.