I was re-reading through what Anware was mentioning about your geometry and what he said was that your geometry is not fully watertight due to one of the uncapped ends of the pipe as you can see here.
This a problem as when you set the material point of the mesher, it needs distinct areas to differentiate between internal and external flow hence the need for a truly watertight geometry. If you were to run this now you would see anomalies in the results or even a straight up error as the material is applied to the entire domain due to the solver not being able to know between what is inside and outside.
Regardless, I’m still attempting to mesh to see if there are any possible other issues, but I would suggest you plug the hole like you did for the other 2 ends and try to mesh again.
EDIT: I’ve managed to get the geometry to mesh successfully as you can see below. Your error was still likely that of not selecting the entire geometry via the Right Click > Select all function. Either way you can attempt to simulate but refer to my earlier statement about the need to ensure a watertight geometry. The project link is here and I’ll leave it up for you, the mesh is under “Barry Mesh”. Do get the new geometry made and meshed! Good luck!