"Coffee is free if you are having the fixed-price menu of the day."
Translation:Le café est offert si tu prends le menu du jour.
It's either prix-fixe or du jour. A prix-fixe menu may be the same every day. The menu du jour, also prix- fixe often changes every day. This is a trick sentence. Culturally how are native English speakers who are unfamiliar with French menu styles supposed to know that 'fixed price' is understood in 'menu du jour'. It's very irritiating when you're one question off a 'perfect'.
*Du café est offert si vous prenez le menu du jour. Does the subject have to be the definite le café? If so, I don't know why it cannot be the indefinite noun phrase introduced by the partitive du. There was a similar expression in the course: Une bouteille est offerte si vous en achetez deux, where the subject is an indefinite noun phrase.