Sometimes Duolingo accepts the most natural translation and other times it requires more literal translations. It seems that when 의 is used it typically wants you to use something like "the squad of Korea" which is not very natural sounding in English but in this case it wanted the more natural translation of "The Korean squad is the best". A bit inconsistent.
That being said, I appreciate all the good work put into this course.
Even the Korean sentence is not natural. It's better without "의"
: 한국 선수단은 가장 좋은 선수단이다.
Not to mention the sentence itself is very cringe.