The issue, I think, is the "the." It is hard to imagine situations in English when it would be used here. It would have to be one particular coffee ("I like the coffee that she gave me") which is hard to generalize to a phrase so obviously discussing a habitual, repeated action.
Maybe you explanation is not entirely correct. I think that it should be acceptable to translate this sentence 'Coffee after dawn is pleasing me'. The Latin verb in this sentence is 'placere' and Duolingo have decided to use the English verb 'to please' in its continuous aspect 'is pleasing' as the only alternative translation. There is nothing in the Latin text to indicate that the writer would have wanted to speak about the pleasantness (e.g. iucunda?) of the coffee that is drunk in the morning but rather that drinking the coffee in itself 'is pleasing' the drinker. The better translation would be 'pleases me', and - since the offered verb is 'is pleasing' that accepts a direct object - the correct translation using the continuous aspect should be 'is pleasing me'.
According to Wikipedia in Latin (Vicipaedia) coffea is the plant, and the beverage is:
cafaeum, cafea, cafeum, caffeum, cofeum, coffea or potus arabicus.
All have to be accepted here, so you can have fun by reporting them.
Here, they write it "caffea", they didn't list this spelling, unless it s the plural of "caffeum", in this case, the sentence is "coffees".
If the question is addressed to me--sorry, I'm not sure I understand it (I'm a humble Latin teacher, not a linguist).
But the Duolingo sentence clearly has a singular noun (caffea), because the verb (placet) is singular. Plural things "mihi placent" (are pleasing to me).
I'm just guessing but "I like coffee after dawn" ought to be accepted. I don't think there is a word meaning "drink" or "to drink" in this sentence. However, I could be wrong. By no means am I a Latin expert.
If you think it really should be acceptable, you can report it using the flag icon. They usually add suggestions pretty quickly though it can take a few days for the changes to come through on our end.