SharePoint Online allows multi-factor authentication (MFA) for it's accounts. Some companies might have a security policy which might require all accounts to use MFA. This however prevents those accounts from being usable as credentials for the migration-center Sharepoint adapter.
Fortunately this can be solved by setting an App Password, which can be used in applications that connect with SharePoint Online. So inside the SharePoint Online Importer configuration, the same username and this new App Password can be used for the import.
Please see the following Mircosoft support link for more details on how to set it up: