There is a rule saying that one-word 'adverbs of frequency' (how often you do something, such as: always, often, usually, sometimes, rarely, never) go IN FRONT OF a main verb, unless that main verb is the verb 'to be' (am, are, is, was, were...). So, you are supposed to say: "I will NEVER do it again", but "I am ALWAYS right".
Of course, this rule (as many others) could be broken, for special emphasis, for example, but, I guess, DuoLingo sticks to it without exceptions.
I have the same question. I think it has something to do with english often cutting out prepositions that other languages still need.
To illustrate, "you will for all times/always remain..." -> "you will (no preposition now) always remain..." If we do the first, it fits the italian better.