Outlook 2010 to Outlook 2021 - Folders Disappear

New Computer Setup

Windows 11 Pro, Office 2021

I copied the Outlook 2010 .PST file to a external drive.  On the new computer I loaded Outlook 2021 and setup the email account for the user.  It downloaded emails from the cloud.  But not the user folders that had been created.

I then went to File - Open & Export - Open Outlook Data File - and I could see the user email box with their folders.  I then moved the folders, 1 by 1, to the users primary email (the new one setup when I started outlook).  This seemed to have worked.  But I noticed that as it was 'synchronizing' - the folder would disappear.  Then all of them would disappear.  Note the folders were under 'inbox' so when i tried to import the .PST it would crash at the subfolders saying something about folders in inbox are not supported by the IMAP server.

What am I doing wrong?   Suggestions for doing this a better way?   Note this is happening on a number of new computers with new Outlook 2021.

As a side note - when Outlook starts I get a error message about the certificate

"The server you are connected is using a certificate that cannot be verified.  

"The target principal name is incorrect."

"Do you want to continue using the server?"  I click 'Yes'. 

Suggestions?

Thanks,

Darryl