A lot of it has to do with the way sentences are listed as accepted. "Some" was originally listed as optional (it's technically not wrong), but the way it was entered made it appear every single time the sentence was displayed. Over a period of years, this gave learners the impression that it was actually required. It is not. "Des" is simply the plural of "un/une" for which there is no counterpart in English.
The problem is that some of us routinely translate the French plural as some because we want to automatically insert it when translating from English to French.
The Duo system is translation exercises not paraphrases exercises. Sometimes leaving out some words from a translation that don't impede the flow of the sentence simply because ...why not leave it out, who cares?.... seems to be more than just inconsistent.
I think that this is not accepted for the moment because the sentence "L'homme a des chaussures laides." implies that the man is wearing ugly shoes. If this is the context that the creator of that sentence was thinking of when he wrote it, then adding "some" may be wrong (I say "may" because I am not a native English speaker).
Although, I think that your answer should be accepted as well because it could work in a different context. For example, it could work if the man talks about the type of shoes he buys, and then his interlocutor makes a comment about his collection of shoes.
Be aware that "des" (here) is only the plural of "une". There is no counterpart for that in English although some people are inclined to translate it as "some". This notion of "some" is almost always ignored in English. Note:
- un livre, des livres = a book, books (not "some" books)
- une chaussure, des chaussures = a shoe, shoes (not "some" shoes)
The BANGS rule is a suggestion not a requirement. It is a way to simplify applying the actual set of rules that govern adjective placement with nouns.
Subjective/figurative adjectives go in front. Objective/literal adjectives go after the noun. BANGS makes it easier to quickly assign their position without a lot of speculation about whether some quality is subjective or objective.
It is an old car in that order because old is subjective to that car. It is describes the car. It is inherent to that particular car or cars.
It is car blue, in that order, because that is objective. It limits the cars under consideration to those cars that are classified as blue. They are classified as such by objective factors and not whether the car is one hundred percent blue. As an object it is blue. There may be other colors highly visible but we limit the notion of the cars being discussed to predominantly blue cars.
If the preceding is hard to follow don't worry. Just use BANGS and remember the apparent exceptions. There are rules about the exceptions but is is just easier to remember the exceptions than try and reason your through it when speaking.
It used to be accepted but not preferred. But it is now specifically rejected in the introductory lessons. This is a surprise choice to make by Duo. It is at this stage that many English speakers have difficulty with the notion of plural un/une.
Penalizing them for demonstrating that they have the concept down pat simply because it isn't necessary to include it when translating some examples even though it is valid, seems pretty strange.
The partitive article - even in the singular - can usually be omitted with no loss of meaning. Sometimes, including it can even cause confusion. I am thinking of the American use of some as an intensifier, as in: "Dude, you got some ugly shoes!" I am writing this from the other side of the pond, so If I have it wrong I'm sure you'll put me straight...