Translation:Understanding who your enemy is is very important.
Since there is no further context here, both "who is your enemy" and "who your enemy is" should both be accepted. "Who is your enemy" is probably better stylistically because it avoids saying "...is is..." There are at least two possible interpretations of the sentence but stress accent rather than word order would be the distinguishing feature. (Knowing which people are your enemies is different from knowing the status and power of your enemies.)
Understanding who your enemy is very important should be accepted duolingo sucks
Sorry, but your English is incorrect. 'Understanding who your enemy' needs an "is" somewhere, as does "important." You can't avoid using "is' twice but you can avoid saying "is is."