Why is my Office 365 Calendar Not Syncing With Outlook?
The problem of Office 365 Calendar Not Syncing With Outlook is quite prevalent. Microsoft Outlook 2016 and Outlook for Office 365 use a more modern approach to authentication. The current structure will be saved on the server if you try to access the Internet calendar file (even if it doesn't require authentication).
While most servers effectively enable these features via contemporary authentication, others consider the client request illegitimate. Another possible cause is a problem with your PST file, which prevents Office 365 calendars from syncing with MS Outlook.