For whatever reasons, US English is comfortable with "in line" and "on line", when the meaning suggests a specific line. "In the line" might seem more logical in many contexts. We don't say "in hospital", as the British do, we say "in the hospital". Yet "in the line" is less likely than "in line". Of course, it depends on the specific sentence and intended meaning.
Esperanto is more likely to use "en la vico" than "en vico", when the meaning is "I waited in line for the bus", "there were many people in line at the theater that night", and phrases of that sort, that indicate a specific line that will be processed from one end to the other in sequence. Without a specific context and meaning, it's misleading to offer a general translation.
If you Google "waiting on line or in line", you will be offered roughly 2 million websites which discuss this question. "Waiting on line" brings up 700 million sites. So while Draquila has missed out up to now, hundreds of millions of people are aware of the "waiting on line" option. "Waiting on line" shows up in plenty of movies and books, and is associated with New York, and to a lesser extent with New England.
Well, some people consider New York to be part of the USA. :-) I first heard the expression "waiting on line" when I moved to Boston as a teenager. As one example of movie usage, here is a link to part of the screenplay from The Godfather II, which contains a couple of examples of that phrase. http://www.jgeoff.com/godfather/gf2/transcript/gf2transcript.html
I've never, in my life, heard "on line". The closest iteration I've come across is "He drums on the drumline".
Still, those iterations are always accompanied by "the". If I were to hear someone say that here in Texas, my first assumption would be that their first language wasn't English.
Certainly, cockneys of the Empire State don't have the best grammar, so if that is where the condensed use of " on line", therein might lie your answer.
Actually, you can tell which part of the Empire State a person is from by whether they wait "on line" or "in line." I'm from the part of the state that says "in line." While not as big as Texas, New York is a pretty big state - and more diverse than can be picked up from watching sitcoms and prime time cop shows.
This comment is a year old. I've made some friends around the world since then. A lot of people make broad assumptions about Texas and its inhabitants, disregarding its size and diversity as well.
I later learned about other areas in New York from friends, particularly ones that lived on farms. At the time, it blew my mind. But it makes sense. There are still people I've talked to that were surprised to find out that not everyone in Texas is a six-shooter wielding cowboy living in the desert.
Perhaps the system is being picky about "in line" versus "in the line". The two ideas are similar. In some contexts, the distinction might be important, and in others, it wouldn't be. Duolingo never gives us a communicative context, so we can't really judge the intended situational meaning. But since the main goal is learning Esperanto, attention to the details and the global concepts are both important. This is a case where the system got insistent about a detail.
As is often the case in language learning examples, we don't have enough context to make a solid judgment. In real life, there is almost always more context, and often that guides us to the correct conclusion, when hearing a sentence like this. And it often misguides us, too.
I see this sentence as being very similar to the English "She is the first girl in line." Does that mean that she is the first person in line, the first female in line, or the first female below, say, 18 years old, in line? We can't be sure what is intended, and neither English nor Esperanto demands specificity. If the speaker mostly cares about the line, the front of the line, and whoever happens to be first in line, then they are likely to say "She is the first person in line/Ŝi estas la unua persono en la vico." But if the speaker is already thinking about this specific individual "she/ŝi" (let's call her Mary), and the focus in on where Mary is, then it is quite likely that they would say, "She is the first girl in line/Ŝi estas la unua knabino en la vico." On the other hand, we get exactly the same sentences, if we are focused on the genders, and how they are arranged relative to each other in this line.
Every language embraces ambiguity, and clarifies some things, at some times, somewhat capriciously, depending on the speaker and listener. This sentence is one of the many cases where you can't reach a firm conclusion about the intended meaning, without more context or a clarifying question.