This is a lie. Plain and simple. A fabrication. A mendacity. I have done this countless times...they HATE it because it is against the anti customer policies Hyatt increasingly chooses to pursue over the years, but there is nothing stopping you from booking nights 1-5 on a reservation, applying a DSU, then manually booking night 6 online with your free night cert. If you try and apply the DSU while both reservations exist, the reps will refuse to apply the DSU and will be hostile about the entire thing, despite there being ZERO reason to not let you use it, beyond some nonsensical idea that you will not accept the possibility that the hotel does not have to and may not keep you in the DSU room for the free night. There is no actual reason that prevents it however with regards to technology supposedly detecting the free night after the fact being adjacent to a DSU applied reservation and then merging them sua sponte Skynet style and deleting your DSU. One of Hyatt's hands down most asinine rules that exists for one reason and one reason alone...increased breakage of awards and upgrades by reducing their utility and value.