Should "I couldn't do it" work? I don't see why it's "this", since it's "es"
I translated "Ich könnte das nicht machen" to "I cannot make that" - it wasn't accepted, Duo wanted "could not" - is this past tense? I tend to use Duo on a smartphone so don't have access to the lessons, just the exercises, so sorry if this is already explained somewhere..
The "konnte" (umlaut over "o") in the sentence is the subjunctive form of "konnen" (umlaut over "o"). That means it should be translated as "could" as in "would be able to". Or in this case, "would not be able to." So, the sentence should--correctly--be translated as "I could not (or 'couldn't'). The verb "machen" can mean "make". But it can also mean "do". It's more idiomatic to translate the sentence,l then, as "I couldnt (wouldnt be able to) do that."
DL states--wrongly--that one of the accepted translations of "Ich konnte (umlaut over "o") is "I wasn't able to do that." That's wrong. There should be no umlaut over the "o" if the meaning is "I wasn't able to do that." Further, "I couldn't do that" = "I wouldn't be able to do that". And translating the sentence using the latter construction is correct and shouldn't be marked wrong.
The verb form showed here is called in German Konjunktiv II.
Link with German explaination: https://deutsch.lingolia.com/de/grammatik/verben/konjunktiv/konjunktiv-2
Link with English explaination: http://en.wikipedia.org/wiki/Subjunctive_mood#Konjunktiv_II