"What am I drinking?"
Translation:Що я п'ю?
"Я" is required here. In Ukrainian it is not like in some languages (e.g., Polish, Turkish) in which personal pronouns can be often omitted. Still, there are some situations in Ukrainian (e.g., set phrases, idioms, colloquial speech, etc.) in which you can also omit personal pronouns, but, as far as I know, there are no grammatical rules for these. Without a lot of experience, omitting a pronoun will most certainly sound unnatural.