To add to Jeff's reply. Here is a way to duplicate the message.
Somewhere down the line, will most likely have been part tree info based on objects (likely 2d) on hidden layers.
For example. open new from scratch drawing.
Turn on ACIS Part tree. Create a new layer, so one has layer '0' and Layer_1.
Draw any size 3d box (ISO view is easier) on layer '0'.
Draw a circle on top of the box (Auto workplane by facet turned on) and put it on layer_1.
Ensure layer '0' is the active layer, and select the Extrusion tool
With compound profile turned ON. Extrude the circle downwards through the box (like one wants to create a hole).
Subtract the extrusion from the box.
Turn off layer_1.
In part tree one has a box + Subtraction based on extrusion based on ccircle.
Select the object and try - create group. you get the message.