Is the object you are trying to 3d-print a “manifold-solid”.
“Entity Info” should report it as being a “Solid”.
It should be a ‘container’ - a group or component-instance - that has the following properties…
It only contains geometry - edges and faces [although non-interactive Guides are acceptable, they are also best avoided].
Every edge must support exactly two faces.
That is, there can be no faceless edges, no edges with only one face - flaps or shelves, or perimeters to “holes”, no edges supporting more that two faces - no internal partition faces with three or more faces per edge, or otherwise solid objects sharing a common edge - e.g. two ‘cubes’ touching at a common edge that has four faces.
Also all faces of the ‘solid’ object should be oriented to face outwards - i.e. you should never see the ‘back-faces’.
Another possible but obscure issue is that although an object might report as ‘solid’ it can be unprintable in3d…
That is if the geometry is self-intersecting - so some faces pass through others.
These cannot be printed.
If the geometry inside the ‘container’ is selected and the context-menu used to intersect with itself results in an object that no longer reports as a ‘Solid’ in ‘Entity Info’, then something is wrong, and you probably need to delete internal partition faces etc…