Translation:I need some good advice but people give me only unhelpful advice.
Actually, дают agrees with its plural subject люди. I believe the confusion comes from the fact that the English word "advice" is uncountable, and therefore has no plural form; replace it with, for example, apples, and you'll realize that the same thing happens in English: "I need a good apple but people only give tasteless apples."
In other words you're right, there is an implied советы.
de nada Lots of words in English can only take the definite article or a phrase of quanitity eg "I want some sugar/the sugar" as opposed to "I want a sugar cube". Compare "How much sugar?" with "How many sugarcubes?"
However, you can say "sugar" but only in the chemical sense ie "glucose is a sugar"
"Water" is similar, but again you can say " a water" in the sense of "Do you want a water?" meaning "Do you want a bottle of water (as opposed to fruit juice)". the whole thing is very idiomatic
"I need some good advice but people give me only useless ones" was rejected too. I guess the problem is not the "a (good advice)" but the "ones". Is it really wrong to say "ones" or "advices"?
EDIT: Singular "one" was marked wrong too. It seems that Duolingo only accepts "(people give me only useless) advice". You can't say "useless one" instead of "useless advice".
Мне нужен полезное упражнение, а люди дают только бесполезные. ))
Sorry, I don't really mean that, I just couldn't resist. But this exercise does have a problem - the difference in structure between Russian and English demands either a literal translation - which isn't accepted - or a free one we can't really predict. ((
Can I advance my usual solution: accept a literal translation, but suggest a better free one? Literally the best I could think of was "I need good advice, and people give only useless", which I think is still acceptable grammar, though elliptical.
П.С. FWIW, Yandex matches my translation almost exactly, whereas Google makes a grammatical mistake: "...useless ones".
The word "advice" is an unusual word. It is defined as an "uncountable noun.".
In this sentence, even though the word is grammatically singular, its meaning implies plural. But your phrase "a useless one" is strictly singular and does not match the implied plural "advice." So, there is a lack of parallelism that sounds wrong to an English speaker. This is why DL's translation had to fix the problem by using the word advice twice even though the Russian only used it once.
This is a inappropriate sentence for DL to use at this level of Russian study.
I got this question as a type-what-you hear, but the male voice sounded as though it were saying бесполезный both times. This was true for all versions: fast and slow on the question page, and the version on this page. On Forvo I can clearly hear the difference between бесполезный and бесполезные.