The Illicit Reaper Robes do not have an ornament slot, this bug has been in the live game for a few seasons now. There has been other armors with this issue in the past that were fixed so I assume Bungie aren't aware of this one.
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For the last time these robes were never meant to have a slot as they were designed for a specific game mode to appear as they are. Bungie said this themselves a while back and it’s not being investigated for this reason