Fillets requires that edges be selected
Workplane.fillet needs a nonempty edge selection. Selecting circular edges from a plain box leaves nothing to fillet.
CadQuery found no Edge objects to round. This precondition failure is distinct from a kernel failure caused by a fillet radius that does not fit.
The selected edge type or direction does not exist on the current solid. Topology may also have changed after a Boolean operation. The diagnostic alone does not establish a radius problem or a disconnected-body problem.
import cadquery as cad
result = cad.Workplane("XY").box(10,10,4).edges("%CIRCLE").fillet(0.5)
Select the four intended vertical edges on this box and apply the original 0.5 mm radius. Check the resulting solid rather than catching the exception and leaving sharp edges.
```python
import cadquery as cad
body = cad.Workplane("XY").box(10,10,4)
edges = body.edges("|Z")
assert edges.size() == 4
result = edges.fillet(0.5)
```