A cloud-based service included in Microsoft 365, delivering scalable messaging and collaboration features with simplified management and automatic updates.
You cannot use this method with Exchange Online. You can however use a script based on the EWS or Graph API to import calendar items from a file. Here's a sample script you can adapt: https://github.com/David-Barrett-MS/PowerShell-EWS-Scripts/blob/master/Legacy/Import-CalendarCSV.ps1