First in the wrist ..the middle row ..if you go to vertices and then show them ..you will see that the whole middle row is not joined ...click on one and then use the move tool and move it ..see how it is not joined ?

you have to do that on all the middle ones ..merge the vertices and you will see the dark edge go away after you done that all the way around the wrist . After you got that done you will be able to convert to sub d but then have the finger issues ...and i showed how to solve that ..