I don't understand why it would be wrong. картошку is Accusative, so it's a direct object of "boil", and as such would be (based on translations in many other exercises) be "the potatoes"
Where does "some" comes from?
Other exercises indicate that "some potatoes" would be Genitive картошки. and that the correct translation here is "the potatoes". In particular, there is:
Купи картошки и свари её.
where genitive is used to say "Buy some potatoes"
A moderator really needs to address this issue.