Looking straight on, the top of the area that is open is curved. That’s ok, but, if you look down on it you’ll see it’s curved that way too, which means the face you’re trying to make would be going in and out of coplanar.
A quick fix would be to draw a line from the left vertical of the gap to the right vertical of the gap. That would fill it in with a flat coplanar face, but you would notice that it doesn’t quite join with the existing curved top edge.
Another thing you could do is draw vertical lines down from each endpoint of the top curved line, that would make the bottom line of the gap have the same top down curve to it.
That’s what I did with the attached file.Kvalve top half - 10X step 3.skp (1.9 MB)