Floating fixtures
A fixture in the air with nothing carrying it is reported as a warning. In the plan it looks like it hangs on the truss; in the scene it does not, and it will not move with the rig.
What is reported
Not attached — won't move with the rig: a fixture that is not mounted on anything and sits higher than the floating fixture threshold.
A fixture standing on the floor is unattached too, and that is fine. Only a fixture above the threshold is reported. The threshold is a project setting: 0.10 m unless you change it, adjustable from 0 to 1 m.
Where it comes from
It was dropped with Truss snap off, or further than about 60 cm from the truss.
It was taken off its truss: Truss snap off, then dragged.
Its truss was deleted. Deleting a truss never deletes its fixtures; they stay where they were, no longer attached.
Fixing it
Mount it. Turn Truss snap on and drag the fixture onto the truss or pipe, within about 60 cm of it. For a stage, riser or set piece, it is Prop snap: which toggle you need depends on what the fixture lands on, not on what you drag. See Build the rig.
Mounting seats the fixture on its carrier and leaves its aim alone, so it does not disturb a focus.
Why it matters
A mounted fixture moves and turns with its truss. One that only looks mounted stays behind when the truss is trimmed, moved or turned, and the plan quietly stops matching the rig.
Mounted on something that no longer exists
A related error: the fixture's mount points at an object that is not in the scene any more. It is rare, because deleting a carrier normally takes its fixtures off it. Fix it the same way: mount the fixture again, or take it off with the snap toggle off.