Hi shokan, also see here for some previous discussions on how the curves need to be arranged for Network to be able to use them, some of these have some illustrations that can help to explain it:
Sometimes it's just a matter of breaking up your existing curves and then joining them together in an arrangement that will suit what network needs.
But if you have something like a sharp feature like it looks like you have for the bottom flat face, that's probably better to just completely leave out and build as a separate step (probably use Construct > Planar if it is supposed to be a totally flat face) rather than trying to include that directly into one network.
