Thanks for posting your query on Microsoft Q&A.
Can you please share your complete scenario to help us understand your query better. For e,g. when are you seeing this error?
Also, which Azure services/products it is related to?
You have added Azure cyclecloud tag, wanted to confirm if this issue is related to Azure cyclecloud in any way?
Few Recommendations:
1.The error you mentioned can occur when an object was converted from a synced user to a cloud-only user, however, the user may not have been deleted properly from the on-prem AD. Is that the case with you? There are two options for resolving this properly.
Option a:
Identify the affected object in Azure AD. Confirm if you really want to delete the object. If that's the case, simply use PowerShell to permanently delete the object from Azure AD.
Run the cmdlet : "Connect-MsolService" and enter your global admin credentials
Next, run : "Remove-MsolUser -ObjectId "<user's object ID>" -RemoveFromRecycleBin"
Trigger a delta sync by running the cmdlet : "Start-ADSyncSyncCycle -PolicyType Delta"
Option b:
If the object has been deleted in Active Directory but you want to keep the "Cloud-Only" object in AAD, simply use PowerShell to clear the SourceAnchor / ImmutableID from the object.
Run the cmdlet: "Connect-MsolService" and enter your global admin credentials
Next, run : 'Set-MsolUser -ObjectId '<user's object ID>' -ImmutableId "$null"'
Trigger a delta sync by running the cmdlet : "Start-ADSyncSyncCycle -PolicyType Delta"
2.If the error appears on the Export step to the AAD connector on Azure AD Connect:
This is likely caused by the account being restored after being deleted by the Azure AD Connect service, before it could confirm the account was deleted. In this case, delete the cloud object and restore it after Azure Ad Connect has synced and confirm the deletion.