Cannot create subfolder in Exchange 2010 mailbox for user in Exchange Shell

Hi;

I want to use Exchange Shell to create a set of subfolders in user's mailbox but not working.

New-MailboxFolder -Parent lastname.firstname:\Inbox -Name CreateFolderTest

My user account is on the OU like mycompany.local\Branch 1\user

it always says, the specified mailbox does not exist whenever I run the syntax with my administrator logon on the Exchange server.

But, I can create the subfolder to the administrator account mailbox when I logon to Administrator account.

I can use this command to list the existing folder in user mailbox.

Get-MailboxFolderStatistics-Identity user | ft Name,FolderPath