Translation:Look, there is still one piece of chocolate in the box.
In case anybody is wondering, I would expect the course - for sentences like this one only - to accept da or de.
I just read the linked article, and since it doesn’t specifically refer to it, I assume you mean:
Rigardu, estas ankoraŭ unu peco da ĉokolado en la skatolo.: there were several pieces of chocolate, and maybe you thought they were all gone, but if you look, you’ll see there’s one left.
Rigardu, estas ankoraŭ unu peco de ĉokolado en la skatolo.: there were several things in the box, including chocolate, and there’s still a piece of chocolate left—perhaps it was the only piece of chocolate that was ever in the box with other things. (Or in this case, maybe “a piece of the chocolate” would be better. And in fact, you can say …*de la ĉokolado, too, but you can’t ever say …*da la ĉokolado.)
And so, you’d offer someone “pecon de ĉokolado?”, not “pecon da ĉokolado?”, but maybe then if they said jes, you’d offer “unu pecon da ĉokolado, aŭ du?”