"Will you clean the coffee maker that is making bitter coffee?"
Translation:Tu nettoieras la cafetière qui fait un café amer ?
That is grammatically correct but it has a slight difference in meaning from the suggested sentence. "qui fait un café amer" suggests the machine "makes a bitter coffee" i.e. the coffee it makes is bitter. "qui fait du café amer" suggests it is currently making some bitter coffee.
You'd usually say the former (duo's phrasing). https://forum.wordreference.com/threads/fr-un-caf%C3%A9-du-caf%C3%A9-le-caf%C3%A9.1203436/