"The general is the leader of the army."
Translation:Ο στρατηγός είναι ο αρχηγός στο στρατό.
Both are correct. In this case,they are interchangeable, since there is no serious context issue. It's added as an alternative and it should be accepted, you can use either of them. ^.^
Keep in mind though, these cases are few. Genitive and accusative are not always interchangeable.