I drink a coffee
Je bois un café - translates literally to I drink a coffee, but appears uncommon or even incorrect usage in English. It should be "I drink coffee", however if this is entered in the tests, it comes back as a wrong response. It could be translated I drink a cup of coffee, but the tests should accept "I drink coffee."
It sounds unnatural to my ear, because the correct translation of "Je bois un café" is "I am drinking a coffee". In English, this clearly indicates that the action is happening now and is on-going. In French, you cannot use the present participle in this way, you just have to use the present tense "je bois". And it is not good practice to introduce the use of an imaginary cup to make the English sound better, since a lot of French people drink their morning coffee from a bowl.
What everyone is saying is correct, however, 'I drink a coffee" does sound unnatural doesn't it.
Perhaps it's more usual to use A COFFEE as in "I'll have a coffee" in response to the question, tea or coffee? it's quicker than saying " a cup of coffee" and quite natural English.
"I drink coffee" would translate as "Je bois du café". "Je bois un café" means "Je bois une tasse de café" or "I drink a cup of coffee"
The test doesn't accept "I drink coffee" because "un café" is a cup of coffee. If you wanted to say "I drink coffee" it would be "Je bois du café."