Outlook Data File – The file C:\Users\<username>\AppData\Local\Microsoft\<username>\username.ost is in use and cannot be accessed. Close any application that is using this file, and then try again.

Fix Cannot open Outlook window. The file .ost cannot be accessed, You must connect to Microsoft Exchange before you can use Outlook data file.

What data file types does Outlook support? While working with Outlook, you will encounter two data file types: OST and PST. OST (Offline Storage Table) file types allow you to work offline.