What is incorrect with "Yksi litra maitoa?"
It is not incorrect. People just say "Litra maitoa".
How about "maitolitra"?
How come you put an "a" after kilo but don't after litra? Shouldn't the case be the same or is that not how it works?
Isn't a liter by definition something countable/measurable? Why is maitoa in partitive?
Partitive is used after expressions of quantity. So A liter of milk becomes litra maitoa.