"Of the two girls, she is the youngest."
Translation:Van de twee meisjes is zij de jongste.
In this case, "het jongste" wouldn't make sense, but I see where you are coming from. For many adjectives, you can also use "het" + superlative (even for de-words), but the meaning is slightly different.
To illustrate this, let's use a different adjective:
- De boom is de grootste - The tree is the largest (of all the trees in the forest)
- De boom is het grootst(e) - The tree is largest (when it gets enough sunlight)
In the second sentence, the English translations doesn't have an article with the superlative. This is the case when we are comparing the size (or another quality, for different adjectives) of the same object in different situations or under different conditions. On the other hand, in the first sentence we are comparing the size of one tree with the sizes of the other trees.
In this sentence here, it is clear that we are comparing the ages of two different girls, so we must use "de jongste" (and the youngest in English).
Frankly, this doesn't apply in at least 100 other sentences here in Duolingo. Actually, word order is changed very frequently here without even a blink of an eye. So it should be accepted here too. It's just annoying how many false errors you get here anyways (at least three per day)
The two words are not completely interchangeable. You must use "zij" rather than "ze" in so-called "emphatic" positions.
Here we are making a comparison regarding two people. Not that one, but this one. Such a comparison by its nature calls for singling out the "she" with the emphatic pronoun "zij".