I wanted to update my Exchange Server using the latest CU19 which had the ZeroDay patch in it. Unfortunately I failed saying that Exchange was not installed and there were no mailboxes. Fortunately MS provided a special KB5000871 patch for Exchange 2016 CU13 several days later. This installed without any problems.
Is there a special way of patching Exchange Server 2016 Edition "Coexistence"? I would really like to get this up to the latest CU level.