A user, email@example.com, organized some meetings in the past. He left the company 6 month ago. His mailbox is deleted since 5 month. But now his meetings should be cancelled. Environment is Office 365.
You cannot use the cmdlet if the organizer mailbox doesnt exist anymore. Instead, you will have to remove the items directly from the attendee's mailboxes. You can use some EWS based solution for this, there are sample scripts available on the internet.