It actually could make sense in the proper context, however currently unaccepted "the girl who is sleeping" would be much more common.
"I'd like to provide some snacks for the children."
"Thank you, most of them like peanut butter and jelly, but the girl who is sleeping eats gimbap."
I can think of several ways to say "a bread".
Sourdough is a bread, as an example. (I concede that many prefer to say a type of bread)
A better example would be sand, or water.
In linguistics, count vs uncount largely depends on if a word is defined by volume or not.
That's a rough and incomplete way to think of it, but works in most cases. (Blood, sand, water, trash, etc all fall under this)
The quick test is if you can pluralize it in this construction, "How many ___ do you have?"
If you can and it doesn't sound weird (aside from certain words making it a weird sentence ;) ), it is almost certainly countable. Otherwise, uncountable. (Bloods, waters, sands, trashes, etc all fail this test of acceptability)
There are some exceptions to this, but these are exceptions from dialectal variation. A quick example is how some dialects can say "fishes" and it won't necessarily sound weird to them, but that group is rather small as many would prefer currently to keep fish as its own plural.
It's nice to inject a little humor into the course on occasion, but the developers seem to have taken pains to introduce ridiculous example as a general practice. I am hoping they will make some significant changes and opt for more useful examples before the course comes out of Beta.
For that you should also get yourself a phrasebook. When I used to go to Korea there was no Duolingo and I could get useful phrases from my phrasebook, but the phrasebook was not very good for learning how to make and understand random sentences. Duolingo is good at that.