You have Active Directory groups being sync’d to Azure AD via Azure AD Connect. These sync’d groups are being used for assigning licenses in your tenant. If you delete one of these groups from your Active Directory, AD Connect throws an error and you get an error alert email.
The error says:
The cause of the error is not clear. This operation will be retried during the next synchronization. If the issue persists, contact Technical Support.
If you open the Azure AD Connect Synchronization Service, you can also see the error there:
If you try to delete the same group via PowerShell, you get the following error:
Remove-AzureADGroup : Error occurred while executing RemoveGroup.
Message: Group deletion is not allowed.
Azure AD blocks group deletion when the group is being used to assign licenses. This is to help protect you from accidentally removing all of your users’ licenses with a single action.
Remove the license assignment from the group and run the sync again. You can kick off the sync from your AAD Connect Server by running: