Well you need to provide the ID of the folder, not its name. You can obtain the ID value from the first cmdlet, Get-MgUserMailFolder, it will look something like this: AAMkAGU2MWM5NzU0LWY3MmQtNGI3OS1hNDVlLTBkMzI3OWVlNWIzZgAuAAAAAAChKSJAhlnUTIHtKSso30ThAQBIPfDMxyP-RYhY8M8xmAPVAAS8XYnlAAA=