Translation:He works too slowly to be useful to us.
I agree. Sometimes duo wants literal translation and sometimes it doesn't. In this respect it is sometimes a guessing game. In a classroom situation I am sure a teacher would mark that answer correct. So take it stride and keep going.... you are still learning. I find the hardest part of learning French is thinking in French. They do not form sentences the same way as we do in English.
In English, "He works too slowly for us to be useful" is an ambiguous sentence - i.e. even though it is grammatically correct, it has two possible meanings. The first meaning is that because "he" works too slowly, "he" is not useful. The second meaning is because "he" works too slowly, "we" are not useful. I believe Duolingo has decided to make the ambiguous translation incorrect because it's not as good as a non-ambiguous (i.e. clear) translation.
The sentence seems to imply something slightly different than the translation. The French sentence seems to say that "he" works too slow for the people that are watching him (the beholders), thus making the beholders say that he is not useful to them. The English sentence however seems to be way less specific to who he seems too slow, only that he is useless to the beholders.
Maybe I dig too deep into this or maybe I am not correct at all (I am Dutch natively, not English), but this sentence confused me.