The clean up of the Mailbox Export Requests is usually overlooked on the vast majority of organizations, and sometimes the user that was once exported is removed from the Active Directory. Then, when the administrators remembers to clean up the Mailbox Export Requests, the following error message You must provide a value for this property can be displayed and the request cannot be deleted.
The error described in the introduction of this tutorial can be seen on the image below.
In order to clean those obsolete requests is using ADSIEdit.msc, open it and open Configuration. Expand Configuration, Services, Microsoft Exchange, <organization name> , Mailbox Replication, and MailboxExportRequests. Select all obsolete entries from the right side that you can identify, and click on Delete
The recommendation is always trying to remove using the orthodox method which is using Remove-MailboxExportRequest however if the mailbox does not exist, then the ADSIEdit.msc becomes a viable option.