"It is a new code."
Translation:Es un nuevo código.
According to https://www.spanishdict.com/guide/adjective-placement "neuvo" is one of the adjectives that changes meaning, but given that there is no context, I don't see why Duo shouldn't take both versions. In fact given that "new' is the typical meaning of "nuevo" it would seem that "es un codigo nuevo" would actually be the more correct translation. Makes me wonder what the translator who entered the sentence was thinking. Further adding to the confusion, all 3 translators on spanishdict.com gave the same translation as Duo, although I got mixed answers when I changed the noun to things like bottle, dress, rock, shoes etc. And then https://www.realfastspanish.com/grammar/spanish-adjectives gives essentially the same rules as spanishdict.