I thought milk was in katakana.
「ミルク」comes from the english word while 牛乳「ぎゅうにゅう」means literally "cow milk".
When I visited Japan, milk was usually labeled ミルク
It doesn't accept ミルク.
I think it can be in both ways because I've seen ミルク too.
Is を necessary to go with ください? I feel like other senrences didnt have it, or am i mistaken?
If I recall correctly from class (not native, sorry) it's something of a politeness thing. I was taught it was acceptable to drop を and other particles when you are being very informal.
I think when spoken many people leave it out
「牛乳をください」 should be accepted. I do not understand why it was marked wrong. Reported on Oct. 27, 2017.
牛乳を下さい Was not accepted. >:(
Duolingo should accept "ミルク" as a translation for milk.