There are the following errors by migration of public folders from On-prem to Exchange Online:
"No mail public folder was found in Active Directory with OnPremisseObjectId or LegacyExchange DN. This may indicate that mail public folder objects in Exchange Online are out of sync with your Exchange deployment. You may run the script Sync-MailPublicFolderrs.ps1 on your source Exchange server to update mail-enabled public folders objects in Exchange Online Active Directory."
"Unable to apply folder hierarchy changes for folder."