[Solved] 3D Meshing
This will be a good place to start if you want a hex mesh. If you just want a tet mesh you will need an enclosed solid, meaning that there are no openings. What you are currently showing are surfaces. You will need to first enclose all of the openings using Geom -> Surfaces -> Spline. Once everything is enclose you can go to Solids -> Boundary Surfaces and select all you surfaces which will make 1 large solid. You should then be able to perform a tetrahedral mesh like you are showing.