Edges are not dissolved I created a plane and extruded some edges from it on edit mode. As you see the screenshot, I filled the last two edges to make a rectangular donut. Then I selected all inner edges and tried to dissolve them. But some edges were never dissolved.

How can I accomplish it?
You can’t remove the last two edges. Removing them would leave a face with a hole in it, which Blender doesn’t support. Removing just one of them would leave a face that shares an edge with itself, which Blender also doesn’t support.