This means that you have setup the project to be internal and the internal region is not fully blocked to the outside. There is a gap or a hole somewhere that would, in essence, let water out of you filled this system with water. It needs to be plugged before the analysis will proceed.
There is a tool in Flow Simulation that can help find the leak though called "Leak Tracker". This video goes through where to find it and how to go about using it to find your leak (GoEngineer - Flow Simulation: Leak Tracker).
Thank You Sir For Helping..!!