Especting us to hear and write expressions we have never been introduced to
...is still frustrating, and by putting it down as an error it makes it feel like a waste of time. Latest case: "Il est à sec". I'd never read or heard this expression before, yet was expected to be able to write it down when hearing it. It would be less frustrating if we got to at least see it written first.
Yes, it is pretty annoying. I usually earn at least 90+ duolingo points every day (the full stack of coins), and it is much harder if I choose to learn something new rather than practise the learned units. Sometimes I almost go through the lesson and fail at last tasks once or twice before I can pass it successfully. I try to console myself by thinking that I learn anyway, even if it seems frustrating :)
It remains frustrating being faulted for something you could not know because it has not been introduced properly. Especially when it is an expression (idiom) which is very hard to get from a dictionary like "il est à sec." I did remember it was a situation where something was missing and this lacking was unpleasant. that I associated with sex and money. Well DuoLingo keeps sex out of the lessons mostly, so it had to be money.
For natives like me, it is similarly difficult to translate French idioms into English, which has to be done anyway. get it wrong often, but when I get the correct answer, I try and remember it at least at short term, to do it right next time the same idiom. comes back. When it comes to translations, I can only rely on other translators (anglos of course) to check if my translation into English is good enough. If the text has not been translated before me, I only have to assume I'll never know how right or wrong I was.
But the site can change the system to introduce these idiomatic expressions like they do with new words. That would be way more helpful than give first a question with such expression being given out of the blue.
In addition, I'm not sure this expression is frequently used in this sense. I googled it and the first result was this one. Google Translate gives "it is dry" for this translation which also makes sense to me but Duolingo doesn't accept it.