why Aku and not Saya here ?
Duolingo wants to introduce you to informal words but has the same meaning. But if you want to make a conversation, better use SAYA
saya and aku should be interchangeable in this. there isn't some other part that establishes formal vs informal.
Yes. Was your answer marked wrong? You probably made a mistake somewhere else.
Why not Aku suka tomat jus
It must be "jus tomat" because the main noun is "juice", not "tomato".