In that sentence and lot of others an indefinite article can pop up absolutely randomly. Sometimes you mark it as a mistake. In next exercise, when you translating the sentence as it was suggested previously by your system, you also mark it as a mistake, but now you suggest that the first suggestion, that wasn't count is right!
I agree with the commenter several below. Fish is coming is correct if used in terms of food. Since so many of the comments are food related (i.e. meat and cheese, beef, wine, etc.) this should be a proper solution and perhaps the only correct solution, but certainly not incorrect.