The most common reason is an oversized or large PST file, which is prone to corruption due to integrity issues. Since there are a number of reasons due to which PST files can get damaged/corrupt, users seldom have control over it.
Outlook conflicting software or faulty Add-ins.PST file backup running in the background.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.
