my outlook didn't to converting formats ost to pst

Since this involves Outlook and therefore has nothing to do with Dell systems specifically, this might be a better question to ask in the Microsoft forums, but some preliminary research involves that this conversion is specifically NOT meant to be allowed for security reasons -- but there seem to be third-party tools that may be able to do it.  Or if you can still access the OST file within Outlook because you still have connectivity to the Exchange server, then just use the built-in Archive feature to move them to a PST file.