The translation for, "You drink milk and I drink water" is "Tú bebes leche y yo bebo agua." But if you drop either pronoun, it is marked wrong with a "missing word." Can't tú and yo be left out if the verb made them clear? .
strictly speaking you don't need to have 'yo' but in this case it is used to differentiate your preference as opposed to your friends, that is yo is used for emphasis, as indeed it would be in spoke English. You drink milk buy I (emphasised) drink water.
❤❤❤❤ is right. When you get further on in the course, you can leave them out. At least, that's my experience. I think it may be wanting you to remember and practise the subject pronouns first before you start dropping them. That way, it's easier than dropping them first then learning them afterwards. At least, that's my opinion.