The sentence "J'ai voulu cette voiture" was in the mobile V.P. Imp. (passé imparfait) exercise. That is the passé composé form of vouloir. Why is it there? There were other occurances of passé composé verbs as the correct answer in that exercise. I am confused.
In my estimation, there is no distinction between passé composé and imparfait in Duolingo. Lack of context is the main reason. When one is presented an English sentence in the past tense, one can choose either tense for translation and would be correct.
I'm not sure how the lessons are put together, but I'm guessing that if an English sentence was originally intended to be in the passé composé, there's no reason why it wouldn't be included in the imparfait lesson either. One possible reason this can happen is that a sentence in the PC lesson had been translated in the imparfait correctly, reported, accepted, and now included in the list of correct translations. This can work vice versa. Another reason is the sentences may not be grouped into a single lesson, but the words are. For example, "voulu" appears in the passé composé lesson, but its translation "wanted" can appear in both PC and Imp lessons. Hence, the mix.
This is all conjecture. Someone with better knowledge can explain this better.