Microsoft.Exchange.WebServices.Data.ServiceRequestException: The request failed.
The remote server returned an error: (401) Unauthorized. ---> System.Net.WebException: The remote server returned an error: (401) Unauthorized.
Note: The following error can be found after enabling logging on the settings of the Teams Importer.

This occurs when the Office 365 Exchange Online permissions: full_access_as_app; MailboxSettings.Read permission has not been added.
As of the writing of this technote, adding the permission full_access_app; MailboxSettings.Read permission requires editing the Azure App Manifest file.
Please review the latest Merge1 User Guide - Microsoft Teams - Microsoft Azure App Creation section for additional details on required permissions to create a Microsoft Azure App in Microsoft Azure Active Directory admin center for Microsoft Teams.
