'Need milk' isn't correct English. 'Do you need milk?' is correct, as shown by Duo. An English sentence always needs a subject pronoun, which on this occasion is 'you', and the auxiliar 'do' as it is a question.
When the voice said necesitas leche, there was no inflection to indicate that it was a question. How are we supposed to know without any written indication or voice inflection that it is a question versus a statement?