The error message explains the problem: the BooleanProcessor, which converts volumes to mesh objects, has failed. To overcome the issue you can:
- use built-in RayTracer;
- or to draw the collimator as a cloud of points:
/vis/viewer/set/style cloud
In reality, neither G4MultiUnion nor G4SubtractionSolid are not needed to create a geometry like yours. Hexagonal holes can be defined as child volumes of the disk. A detailed explanation of how to do this is available in the thread you mentioned: Need advice how to create Hexagonal prism