Translation:I don't buy a lot of clothes with nineteen pesos.
It's not grammatically incorrect, but it certainly is something I'd be likely to make fun of a native speaker for saying...
At least to me, the English sentence seems to be more about a choice than an ability. "I choose not to buy a lot of clothes with 19 pesos." rather than "I can not buy a lot of clothes with 19 pesos."
Which certainly seems odd to me.
Dizzy, you're right. It should have been accepted. "Many" applies to countable objects; "much" applies to uncountable objects. It's not a matter of singular or plural, as another response claimed.
"Clothes" can be treated as an uncountable word. Although, it's kind of a weird case, because "items of clothes" is countable, but not "clothes". To illustrate: "three items of clothes" is grammatically correct, whereas "three clothes" is not. Another weird example that further illustrates this is "peas". They're countable in the strictest sense, but they aren't measured or commonly spoken of in that way. For instance, no one asks for 100 peas, they ask for a spoonful. Because this is the case, "peas" can be treated as uncountable. "I ate too much peas" is a grammatically correct sentence.
Notice that in both cases (i.e. "clothes" and "peas") I say they CAN BE treated as uncountable. They're both strange cases, and they don't necessarily HAVE TO be treated this way . There's a lot of debate over whether "many" or "much" should be applied to them, because there's confusion over their countability. Unfortunately, sometimes language doesn't fit nicely under a strict set of guidelines. There will always be cases where grammar and word usage will be unclear... But that just makes it more fun!
In any case, "much" should be as an acceptable translation as "many", and given the complexity here, Duolingo should have accepted your translation. Although, I will say "a lot" sounds the best of the three.
If I'm being unclear here, or if you want to read more about this, I found a couple good threads on the topic.
For the thread on peas: https://english.stackexchange.com/questions/187625/are-peas-countable-or-non-countable
For the thread on clothes: https://www.englishforums.com/English/HowMuchOrHowManyClothes/bvdrh/post.htm
The point of the exercise is not to teach us how sentences are usually put together in English, but rather in Spanish. The translation given may have to be a little awkward in order ato correctly teach a person what the sentence says in Spanish, in a natural Spanish way. For example, "I call myself Steve" would be an unnatural English sentence, but it's necessary to correctly teach us what's natural in Spanish.
Unfortunately, they translate it as my name is, so we have to look to comments to find out it is actually i am called. However, if i enter i am called, duo takes it. I wish duo would do both, exact translation and then vernacular. I find this helpful to understand how a spanish sentence is constructed. Like sentences that have me gusta in them, it really helps to know its exact translation is it pleases me.
I feel like in English, this would be expressed with "19 pesos doesn't buy many/much clothes"
Also it's obnoxious to make the answer incorrect on the basis of whether you use "clothes" as a count or noncount noun, when that's more of an English issue (nothing to do with whether I understood the Spanish). Same thing when they mark something wrong for typos with "a" vs "an".
These lessons are not meant to provide handy things you might say on a daily basis. These lessons are meant to teach grammar and vocabulary. The course contributors often deliberately create strange sentences to force you to learn the grammar and vocabulary rather than just coasting off of context clues.