Translation:None of them know how to use chopsticks.
Nope, this is not what the exercise asked for.
You introduced 都， therefore，according to your hovers, we have the choice to say either
_"None of them know how to use chopsticks" _
or: "They both don't know how to use chopsticks"
YOUR OWN "CORRECT" SENTENCE on the exercise page says: They all don't know how to use chopsticks.
That's an awkward translation if I've ever seen one. I used to make a living being a translator, and this here "ain't it". . .
You can report it. That way the course will improve. Noone said it was perfect.
Still rejected. Both seem right, though I guess they want one to assume the "cannot" meaning since being able to use chopsticks is a learned skill as described at https://resources.allsetlearning.com/chinese/grammar/Expressing_a_learned_skill_with_%22hui%22.
The term 会 in itself has 14 different meanings being two of them capability and likeness of something happening (will) so in this case context is really the only way of knowing what the meaning is. Probably because in DuoLingo most phrases are thrown at you without context interpretation maybe a bit difficult, but don't be intimidated by it! Mandarin is a beautiful language and in casual settings comunication is actually pretty easy so keep on practicing and you will get good at it!
Yes, there could potentially be two of them. Generally, this sentence means "None of them know how to use chopsticks".
I suppose this must be a sticky point between Chinese and English grammar. "They can't use chopsticks" sounds more natural than "They all...", but seeing that 都 character made me think some sort of quantity needed to be added to the translation.
How would we say 'They can't both use chopsticks' as opposed to 'they both can't.. ?
I think the best translation is, "Neither (one) of them knows how to use chopsticks"
"They can't all use chopsticks" is the grammatically correct version unless it meant that "none of them can use chopsticks". But anyway, Duolingo rejected it for unknown reasons.