Hey Matthew, nice model you have here! Did you build it?
The flow volume extraction operation fails most likely because your model is only made up of surfaces, and the surfaces do not form closed volumes.
I can also see that you already tried a few CAD mode operations, am I right? In order to overcome this, please try going back to your CAD program and cleaning up the model:
- Remove small parts
- Join surfaces
- Create enclosed volumes from the surfaces