Chuck,
New Outlook does not have Offline Mode yet so you will need to be connected to the internet to read the emails.
Since you have M365 it might be best to get Classic Outlook working to use for offline needs. The errors you are getting about PST not found I think has to do with Classic Outlook trying to open but there is no profile.
To create a new Classic Outlook profile do the following:
- Right click the Windows Start button, click Run, and then put this command and click OK: outlook.exe /profiles
- Click Options, New, name the profile and proceed to add an account