Fix: Cannot expand the folder. File access is denied. – Outlook
If you attempt to expand a folder (inbox, pst, sent items, etc.) within Microsoft Outlook, you receive a permissions denied error.
Cannot expand the folder. File access is denied. You do not have the permission required to access the file.
It is likely that a Windows process is locking your Outlook data file (OST, PST). Use Process Explorer to check if a process is locking any Microsoft Outlook files. In some cases, an Antivirus program may be scanning the file.