When trying to empty my Trash I had been receiving this error message for the past few weeks:
To fix the error message "Outlook is synchronizing local changes made to the items in this folder. You cannot remove this folder until the synchronization with the server is complete." I needed to:
Right click on the Deleted Folder and select Properties, then click the button Clear Offline Items:

You may also need to check inside your Delete Items folder and remove other folders that had been created inside your Trash.
I also had to use OWA to delete a folder that could not be deleted using Outlook.
To fix the error message "Outlook is synchronizing local changes made to the items in this folder. You cannot remove this folder until the synchronization with the server is complete." I needed to:
Right click on the Deleted Folder and select Properties, then click the button Clear Offline Items:


You may also need to check inside your Delete Items folder and remove other folders that had been created inside your Trash.
I also had to use OWA to delete a folder that could not be deleted using Outlook.
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