How to solve "model not watertight problem"?

Hi,

I'm a beginner to the flow simulation part of solidworks and i'm trying to simulate air flow in PCB housing, the analysis type is internal, but the pre-processing gets stuck saying the model is not watertight, how do I resolve this?

P.S- Also the 'check geometry' and 'create lids' buttons seem not to be working, do I need a separate electronics cooling module for these kinds of simulations?

SolidworksFlow Simulation