"You have coffee while he finishes his sandwich."
Translation:Vous prenez un café pendant qu'il finit son sandwich.
The system must be accepting the verb boire also. Interesting. It is okay to say both un café and du café but I don't know why the translation uses the indefinite article instead of the partitive article or why it marks the partitive article wrong. You got me there. My opinion is that the article "a" should be included before coffee in the given sentence to avoid confusion.
you can say both "du café" and "un café", the difference is that "un café" is more precise, you take one coffee (we don't really know if it's a cup, a bowl or a bottle but it's one so... it's more precise^^) when "du café" means more "some coffee", you have coffee but we have no idea on how much you have.
So finally, here I prefer the translation "Tu (Vous) bois/prends (buvez/prenez) du café" because you have no indication about the amount of coffee in the english sentence.