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"?
I have never seen German use "als" in similes. The full wording should be "So schlau wie ein Fuchs". German often omits the "So", in the same way that English often omits the "As"; but this practice is an ellipsis. "As" X as Y" is understood, and is preferred in careful speech and writing.
If any moderators do read these comments, I would say please read all the comments about the English expression "as sly as a fox". It is irrevelant to say the German has no equivalent als......wie. To translate " Er ist schau wie ein Fuchs" as "He is sly as a fox" is as wrong as translating"He is as sly as a fox" as Er ist als schau als ein Fuchs.
Whatever the German is, and there seems to be some doubt, the English is a standard common phrase, "as sly as a fox", as SeanMeaneyPL explains clearly above. There is a strand of traditional European animal folk literature where the Fox character is cunning, sly etc. See "Le Roman de Renard le Goupil" for the basic French form. So the fox is traditionally given the character of being cunning/sly.
Sly like a fox is ungrammatical English. Sly as a fox is an acceptable shortening, but is colloquial. The correct structure is "as sly as a fox".
As sly as a fox, as strong as an ox As fast as a hare, as brave as a bear As free as a bird, as neat as a word As quiet as a mouse, as big as a house.
Lenka "Everything at Once"
The figure of speech is called a simile, and has a fixed form, "As X as Y".