"I like juice."
は is sometimes a contrast marker. If the sentence were ジュースは好きです, it would imply that you like juice but not some other unmentioned thing (e.g. tea). が does not carry this connotation.
For example: ジュースは好きです。お茶は好きではありません。(I like juice, but I don't like tea.)
Another example: John and Rose are an interesting couple. Imagine someone asks you if you like them. You might answer: ジョンは好きです。The translation would be "I like John," but since you've used は, you're leaving the contrast unspoken, "I like John (but I don't necessarily like Rose)."
But if が is a subject marker and you're viewing this as a European language, then this sentence would mean "Juice likes." with the direct object omitted, wouldn't it?
Yes, that would be correct. But the way you say you like something in Japanese is quite different syntactically; 好き is an adjective with the liked thing as a subject, rather than a transitive verb with the liked thing as an object.
Devoid of any conversational context neither is really wrong; it doesn't really elucidate the difference but that would be hard to do in the duolingo model : (
Juice=ジュース writing in kanji: 果汁（かじゅう）the meaning is 果物の汁 (果物：くだもの＝fruit,汁：しる＝juice,soup）I like juice. 私は果汁がすきです。(^_-)-☆