Duo is pretty picky about a/an. (You probably already know that you need to use an before vowel sounds and a before consonant sounds).
On the other hand, it automatically generates numbers for all English words that can be written with numbers. So since ett brev means both a letter and one letter, "one letter" is an accepted answer – so the machine generates "1 letter".
Those are general things for all of Duo so we as course contributors can't do anything about it.
Is it really continuous? I read and I am reading are different in English and some other languages. When I read the above and other verbs with a similar syntax, my perception of it, is just the verb, no ~ing. So, jag skriver, seems to me, I write, not I am writing. I think I am wrong, as there maybe no way of discriminating the two in Swedish, by and large, if so, I'll stop worrying about it. Is it so?
Swedish doesn't have a continuous, so jag läser does indeed translate to both "I read" and "I am reading" depending on the context. In the case of reading particularly, "I am reading" is generally better for the course since there is no risk of confusing past and present as with "I read". We do have other ways of expressing that something is continuous, which you'll encounter later in the course.