"coffee" adalah benda jamak, tidak boleh menambahkan "a" di depan kata "coffee""coffee" adalah benda jamak, tidak boleh menambahkan "a" di depan kata "coffee"
You can say 'a coffee', 'a beer', 'a juice', etc.
Example Waiter to solo customer: 'What would you like to drink?' 'Could you get me a juice please.' - one glass of juice arrives
Or: Waiter to a family of 4 people: 'What would you all like to drink?' We'd like juice please.' - 4 glasses of juice arrives. 'We'd like a juice' would imply the four people were sharing one glass, which would be rather strange.
In the case of 'a coffee' it is short for 'a cup/mug/glass of coffee'. It is a singular noun
So I think 'minum kopi' is not a good translation, you should say 'minum segelas kopi' or 'minum secangkir kopi'