Why is my tank not watertight if there are no holes?

Hello.

I was trying to do flow simulation on my water tank, but it says that it is not a watertight volume. I don't understand why because I just extruded a circle and put a chamfer on one side of it. What am I missing?

Attach is part file

Thank you for your time.

I'm using solidworks 2017

SolidworksSimulation