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'