"No veo ningún supermercado."
Translation:I don't see any supermarkets.
18/07/2018 "i cannot see any supermarket" accepted by DL ellos me dijen que fue en esta calle!
Like other questions involving this grammar, duo is making that decision that is natural and equal to put these Spanish sentences into the plural form for the English translations. Duo users are getting tired of this decision. In this sentence/context, for example, we have the present tense. How likely is it that someone is going to be standing on the street and say, "I don't see any supermarkets?" Possible, but highly unlikely. Supermarkets are not typically clustered in large numbers, side by side like trees in a park or forest. On the other hand, if one walked around the city all day and then reported in the past tense, "I DID NOT SEE any SUPERMARKETS," one could say that one was attempting to be "natural" in the translation, even though the plural noun does not match the singular noun in the Spanish sentence.
As you probably know, it is natural (and required) that the word "any" be used with plural countable nouns in English.
"Warning: We don’t use any with this meaning with singular countable nouns: Have you got any Italian cookery books? (or … an Italian cookery book?) Not: Have you got any Italian cookery book?"
On the other hand, it is customary (and natural) for the same sentence in Spanish to be singular unless it's a noun that is usually always plural in Spanish.
So, it needs to be plural in English and singular in Spanish to sound natural in both languages. If you understand that, then you will be able to use ningún in a way that sounds natural with any tense you like in your own sentences in Spanish. If we try to impose the English rules and customs onto the Spanish sentences, then it won't sound natural in Spanish.
Duolingo is trying to teach grammar indirectly, but we all know we won't be using many of the sentences exactly "as is" when communicating in Spanish. Otherwise, we'd spend a lot of time talking about ducks eating rice.
"I don't see any supermarket" is a perfectly natural statement in the midwestern U.S.