Duolingo won't accept the answers "He is as cunning as a fox" and "He is as sly as a fox". These sentences are proverbial in English. The suggested answer "He is smart as a fox" is ungrammatical because it is missing the first "as". In my variety of English (Australian), "smart" is colloquial rather than correct". "sly", "cunning" and "clever" would be better. "Smart" more properly refers to the way someone is dressed.
The original sentence didn't have "so ... wie", so I think the translation cannot be " as ... as". The translation that DL gives moves me to the thought that the original sentence meant to convey the idea: "He is sly -- like a fox is sly." Did you try "He is cunning as a fox", or "He is smart as a fox"?
Structures don't need to be similar between languages. In a comparison, I would always say "X is as (adjective) as a Y". The first "as"is not optional. I did try "He is as cunning as a fox", but it wasn't accepted, nor was "He is as sly as a fox".