Well, the sentence is about 'not wearing it at all, ever'. Violet is just not my colour. Also I'm a guy and I don't wear dresses.
Unfortunately the Polish equivalent of "to be wearing" hasn't been taught at all in this course. Yeah, many sentences would make a lot more sense if they were in Present Continuous. Anyway, see here: https://forum.duolingo.com/comment/27628688
I can understand, that "to be wearing" is not accepted. But if "nosić" has a general meaning, in the translation "I don't wear violet dresses" should be accepted. Because that's the way how you express the general meaning in Englisch. Otherwise it's like Crabbadon says: I don't wear a purple dress.... is only a part of the whole meaning in a context.
I really don't understand when "I do not wear a violet dress" would ever be used in English or Polish. The only reason would be to say I never wear purples dresses in general which would be "I don't wear violet dresses" or I am not wearing one right now which is "I am not wearing a violet dress." I wonder why this clothes lesson is taught the way it is because there is no practical use for these sentences in day to day speaking in either language. When would you use these types of sentences in the way this language lesson is taught in a conversation? It's not your fault obviously, just commenting.
I agree that the sentences in this section are far from great. Whoever created this part of the course forgot to teach "mieć na sobie" (literally "to have on oneself"), which is the equivalent of "to be wearing", so we have no way of teaching that. The rest of the sentences are either singular, and sound a bit strange, or plural and sound good.
I'll change the main translation here to 'the' violet dress, which makes sense.
However, frankly, I don't think it should be. The problem is the current version of the course doesn't teach the "mieć na sobie" (literally "to have on oneself") construction, and this is what should be used for "wearing" at the moment. It should be "Nie mam na sobie fioletowej sukienki". But we accept "wearing" so far because this structure is missing from the course.
as a native english speaker this sentence sounds a bit strange. I know that "mieć na sobie" indicates present tense so I know "I am not wearing a violet dress" would be an incorrect translation, but still it sounds weird.
I feel like "Nie noszę fioletowej sukien" (I do not wear violet dresses), would be a more natural sentence. But I suppose it is a useful grammatical exercise nonetheless.
That would be "Nie noszę fioletowych sukien", the adjective has to be plural as well. And "sukien" is a correct word, but it's a form of "suknia", which sounds more like a wedding dress or a ball dress than something that women wear every day. So "sukienek" for the more usual version.
Yeah, the notion of wearing was messed up in the process of creation of this course... only Present Simple "nosić" (to wear) was introduced and is used here with singular items of clothing which indeed often sounds strange :/