This is really just asking if the argument is valid, and if so, why. The argument is that since economics requires monetary values, and to assign monetary values people must be able to compare costs and benefits of various things, which is not possible for environmental factors. Thus, environmental economics has a core problem, it is impossible to assign values, which is at the heart of economics. This is more or less exact reasoning put forward, that something is essential, the essential thing is missing, so the goal is frustrated.