Non-manifold edges are when more than two polygons in the mesh share an edge. This mesh has a lot of these due to all the interior polys. You would need to use the command ExtractMeshPart and delete all the interior polygonal faces first. In some sections you’ll also need to remodel some polys using the 3Dface command with Vertex Osnaps. I would not recommend MeshtoNurbs here as that will simply make a planar surface from each poly including those on the interior. I’d either use the extract mesh part>delete workflow or remodel it using planar srfs with the SrfPt command and Vertex Osnaps.