"A brother likes tea with sugar."
Translation:Ein Bruder mag Tee mit Zucker.
Gern + infinitive = to like to.... So, if you translate the sentence as "Ein Bruder trinkt Tee mit Zucker gern", the reader interprets that as "A brother likes TO DRINK tea with sugar". That isn't what the DL practice sentence asks for . To furnish a correct translation of the sentence into German, it should be either a.) Ein Bruder mag Tee mit Zucker" or b.) "Tee mit Zucker gefaellt einem Bruder."
Ok, I know what German grammar is, but this sentence without context should be still translated into German with the help of "gern", because it is just the most neutral, and "mögen" or "gefallen" are not so neutral as a verb + gern to describe you likes/dislikes about activities. I believe in the lessons sequence is it clear which verb the duo wants from you, but in a placement test it's arguable. And while you mean that no "trinken" is implied here, is then just "gern haben" correct? E.g. Ein Bruder hat Tee mit Zucker gern.
"Gern haben" is another way to express "to like (something or somebody). Had you written the practice sentence as "Ein Bruder hat Tee mit Zucker gern", DL should have accepted the idiom. But you were asking about "gern + infinitive. " And that's a different construction. As I said, it's typically translated as "to like to (do) something. Check a German grammar if you're still dubious.