How is your environment deployed? On-premise or Hybrid?
Have the mailboxes been migrated to Exchange Online without calendar data? Was the process failed or what?
The official method: Use network upload to import your organization's PST files to Microsoft 365
The command New-MailboxImportRequest is not supported in Exchange online, and we wouldn't recommend any third-party tools because they may not be safe. If you insist on them, some tools metioned here should be better: Third-party migration
If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.