Duolingo is very inconsistent. In previous question the indefinite article was used. I still believe that the indefinite article is omitted when referring to one's calling, employment, or profession, unless said descriptions are modified by an adjective in which case the indefinite article is required.
When you first start studying languages your inclined to try and literally translate everything from your language, which is incorrect and also what I used to do a lot, because you are thinking in your culture/language and probably assuming everyone else does too. All langauges evolved differently, even if/when they share some common ancestry. So you really have to learn to think in that language. Idioms are the most obvious example but even something as simple as how one says their profession, Soy medico vs I am a doctor, or speaking about the weather "hace calor" (It's hot or literally translated to english "It makes heat"), so it's just the way the langauge evolved within that culture. Spanish or English wasn't translated from one to the other as is the same with all other languages.
It's annoying if you're not a native English speaker. I'm Dutch and we say the equivalent of "he is doctor" instead of "he is a doctor", so I'm prone to translate it directly and of course without the article in English it's wrong, even though I really do know what it means. It would be very user-friendly to international users if it were still correct with the article in these cases.
I'm a Northwest native. In the early days of television it was from this area Network TV newscasters were drafted. Here, we NW natives speak crisp and clear, in a style that is refered to as "clipped." Everywhere else in the US there exists strong accents of one form or another, such as drawls, which can include odd soundings of vowels. We NW natives strongly notice this difference. But why we speak like we do I haven't a clue. But I'll tell you, I love it. Wouldn't want to sound any other way.
As for the oddities in the English language, I was thinking the other day about how difficult for speakers of other language it must be to learn English. I am finding it is considerably challenging to learn Spanish but I am seeing that it must be a vastly easier to learn Spanish than English. Spanish rules are fairly simple in comparison. In English, the rules are a total mess as a result of its being a combination of several languages with German at its roots. Nevertheless I can't see how it relates to German at all.
One good thing I know is that there are a vast number of Spanish words spelled the same in English and a vast number with different endings which follow precise rule concerning their change. This should make for an enormous help for English speakers to learn Spanish. I can see how it would be an advantage of natives of other languages who are now learning Spanish to have studied English, even if they have not perfected their studies of English.
In English, the article can be important, especially depending on context. For example, if I said "He is a man", this would mean that the individual, who is presumably male, is a man. If i were to say "He is man", however, this could easily mean "He is representative of mankind, or of all men". Within context, either could often be used and understood, but formal English requires the article when appropriate since the exclusion of it could change the meaning of the noun.
EDIT: An interesting note should be made about my above statements. I specifically and deliberately stated "'He is representative of mankind, or of all men'", which one might notice is missing an article before "representative". This is because the inclusion of an article (i.e., "He is a representative of mankind") would alter the meaning of the statement. Being a representative is someone or something which represents someone or something else. Being representative is the act of representing in and of itself. Whereas being a representative of mankind would connote that one serves as a representative for mankind, being representative of mankind is actually representing mankind. It's the difference between, say, the emissary one might send on behalf of mankind to an extraterrestrial species waiting in orbit, and the idealization or embodiment of mankind as represented by the person or thing itself. It's a subtle distinction, but significant enough to connote a different meaning which, in turn, could affect the overall message.
It's always optional if you are just saying what the person does for a living. If Duolingo marks it wrong if you don't use an indefinite article in Spanish, you should report it.
If, however, you use an adjective to modify the profession, you need to use the article.
I was about to point this out, actually, if no one else did. The relationship is important both from an etymological and mnemonic standpoint. Technically, "enfermera" means "one who works in an infirmary", who is usually called a nurse. Nowadays, however, the term is used to refer directly to nurses, irrespective of whether their occupation lies in an infirmary or in a hospital (however synonymous both terms may have once been).