I think you should also accept the indefinite article for "duck" You would only use "the" if the lion was eating a particular duck. Note that you can use "duck" as the plural form in addition to "ducks". Examples: "He is hunting duck" and "He keeps ducks".
Hai teman! Your sentence is valid. Even using articles or not (without determiners "ini" / "itu", articles are optional), sentences with "a duck" are just some examples allowed in the English translation. When you see the Indonesian sentence without time markers (sedang, sudah, telah, akan), time words or phrases (hari ini, kemarin, besok), counters (sebuah), or numbers (satu, dua), then the English translation could have different tenses, and singular or plural nouns. Please try adding your sentence using the reporting tool.
I am showing here some examples with "a duck" in present tense only:
The / That lion eats a duck.
The / That lion is eating a duck.
The / That lion has eaten a duck.
The / Those lions eat a duck.
The / Those lions are eating a duck.
The / Those lions have eaten a duck.
Also, I will show some Indonesian example sentences with the translation using indefinite singular and plural or numbers, in simple present only:
Singa itu (The / That lion / Those lions) makan / memakan (eats / eat) satu (a / one) bebek. (duck)
Singa itu (The / That lion / Those lions) makan / memakan (eats / eat) seekor (satu ekor = a / one) bebek. (duck)
Singa itu (The / That lion / Those lions) makan / memakan (eats / eat) dua / dua ekor (two) bebek. (ducks)
Singa itu (The / That lion / Those lions) makan / memakan (eats / eat) beberapa (some) bebek. (ducks) / "some" would be optional here, so "beberapa bebek" can be translated as "ducks" or "some ducks"
Singa itu (The / That lion / Those lions) makan / memakan (eats / eat) bebek-bebek. (ducks / the ducks) / the noun reduplication is used to stress the plural form, since there are no plural nouns in Indonesian
Singa itu makan bebek. (Duolingo sentence) would also be translated as: The / That lion / Those lions + eats / eat + a duck / some ducks / the duck / the ducks / ducks / duck. (as you also said)
Note: the Indonesian verbs "makan" / "memakan" means the same, with no conjugation, and "makan" is more colloquial, not informal.
I hope it helps.
Selamat belajar! :)