The "Valid boundary not found" error message may display when you try to generate a 3D freeform space.
Double-check your model to verify that the objects are really enclosing the space. There may be tiny, barely visible gaps between the designated boundary objects in the building model.
For example, if an interior wall is auto-projected to a roof slab without ceiling, and the interior wall extends only to the baseline and not to the outside of the outside wall, the 3D freeform space cannot be generated. In such a case you could create a ceiling under the roof slab, so that walls are projected to the ceiling instead of the actual roof. Or you could extend the interior wall all the way to the outside of the exterior wall. You could also place a sweep on the wall to make the top of the wall flush with the sloping roof slab.
The boundary objects for an associative 3D freeform space must be valid objects for 3D freeform generation. Any other objects such as railings or 2D linework, will not be recognized by the space generation. If your selection set includes one of these objects, it will be filtered out, and the boundary set will be considered not closed. To create a valid bounded space, substitute a valid boundary object for the invalid one.
When boundary objects like walls, slabs, and other 3D geometry just touch each other but do not have any overlap, or when objects intersect only along an edge (to form a line rather than a surface intersection), they are considered open even though the corresponding real-world components would be closed. In such cases, no 3D freeform space can be created. To fix this problem, extend one of the objects just far enough to create an overlap.