Dear Fadi Mohieddine (ACT)"),
Thank you for visiting Microsoft Community.
We understand you are encountering issue that you cannot open a PST file in new outlook .
The .pst file support is not available in New Outlook for Windows. This feature is in "coming soon" status. You can find the same information from Getting started with the new Outlook for Windows - Microsoft Support .
For easy and quick reference, here summaries screenshot for specific .pst file feature and status.
Since New Outlook for Windows is in still development, so many features are still not available. So, if needed, you can revert to old Outlook by Turn off - Try the new Outlook option.
Thank you so much for your precious time and kind cooperation.