Leveling wood floor for laminate wood floor
Those planks look really cupped and separated. Floor leveler probably isn't a good choice; too many ways for it to go through the floor. It needs a overlay with plywood to make for a solid smooth base. Just ensure that the plywood is laminated and properly fastened to the subfloor. The thinner the plywood the flatter the subfloor needs to be, that means sanding and filling the cups in the wood before laminating to it. From there you can level it off easily.