MS Exchange 2016 CU19 Update Error: The operation failed because UPN value provided for addition/modification is not unique forest-wide
The error is telling you that it is trying to modify the federatedEmail built in user in the AD Users OU. It is not able to do this but it is also not a unique name so it cannot add it. This may be because it is not where it should be or it may be disbaled.
See this article for a bit more info on these accounts.
https://www.azure365pro.com/how-to-recreate-system-mailbox-federatedemail-discoverysearchmailbox-in-exchange-2010/