Object Not Found error: Try selecting the TRC listing that corresponds to the item, or the named item listing instead.
Zone objects not visible in viewport: Rebuild collision map.
Reference objects not visible
If using F4 placement method, then no reference object will place itself unless one was selected previously (Issue 6097). Make sure to first use the Add Node button to select and set up the reference object you want to place.
For large reference objects, move the actor outside of the object or further away to view.
Reference object collisions
If a reference object doesn't have collision, first verify that it possesses a collision model from the Zone > Zone Objects > Reference > Library > Add Collision Model button.
Use Zone > Zone Objects > Bld Coll Map button to regenerate collision maps when (1) it doesn't exist for an existing reference object or (2) it exists for a deleted reference object.
Set Popout Box in Reference Library Editor does not seem to hold onto its valus (Issue 6096).
Actors
All actor bones must possess positive coordinates, or the actor will fall through the ground mesh regardless of how it is placed (Issue 6098).