Because noun "milk" is uncountable in English, hence not using a/an here
'This milk is good' is also possible, a submitted for this as a correct answer. Thank you.
"This milk is good" is not an accepted translation because it is a different sentence grammatically. The course consistently distinguishes between predicative ("This IS good milk") and attributive ("This milk is GOOD") constructions of this type. You will run into this often as you move through the course, so it's good to understand it early on.