No teacher here, but surely rubbish is a noun here? In that case an adjective should precede it.
You could have "... it's a completely rubbishy statement" (not sure if rubbishy is a real word, but it's used as an adjective and preceded by an adverb) or "... it's complete rubbish" (rubbish is a noun, preceded by an adjective).
Or did I misunderstand you question completely?
Student or Learner