Improper Outlook termination causing outlook.exe to run in the background.
There could be several reasons that can trigger this error, such as, While it may appear that the PST file is being used by some other application, quitting all applications or restarting the computer might also not resolve this error. Reason for ‘Outlook PST cannot be opened’ Error Close any application that is using this file, and then try again. The file C:\Users\username\AppData\Local\Microsoft\user name\username.pst is in use and cannot be accessed. However, sometimes PST becomes inaccessible and thus, an error such as Outlook PST is in use and cannot be Opened, appears. One can open and access the emails stored in a backup PST file anytime via Outlook options. The PST file format is also used to backup and archive the Outlook Exchange, Office 365, and IMAP emails. from the email server and saves it in the PST file. Outlook syncs the mailbox data, such as emails, attachments, contacts, calendar, notes, etc. The Personal Storage Table (PST) file is created by default when you add or configure a POP account in MS Outlook. Microsoft Outlook stores the mailbox data either in OST or PST file format.