How were we supposed to know it was supposed to be wy and not ty?
You were not supposed to know that, because standard English doesn't show this difference. In 99% of the sentences here, both "ty" and "wy" are equally good answers. "Masz mleko" and "Ty masz mleko" are not only accepted, but starred.
Why does "Du Hast Milch" not work?
Because this is Polish for English-speakers, not for German-speakers?
Oh haha :)
how did you end up in here?
When do you use Masz/Macie/Maja/Mamy?
Take a look at https://en.wiktionary.org/wiki/mie%C4%87 and click the conjugation tab to see the how these words are used. Note that your third option should be "mają" rather than "maja".
Why "Ty macie mleko" isn't good?
Because the conjugation is not correct. You can either say "(Ty) masz mleko" or "(Wy) macie mleko".
Alright, thanks. But have they the same meaning?
"Wy macie mleko" is used for second person plural (i.e. more than one people have milk) and "Ty masz mleko" is used for second person singular (i.e. one person has milk).
There is nothing that indicates a plural "you" in this sentence.
Correct - both plural and singular Polish forms are acceptable here.