"Take care of your body."
Translation:Dbaj o swoje ciało.
"twojej" is either just a typo or a wrong form.
"Dbaj o twoje ciało" is correct, but kinda clumsy.
Whenever a form of "swój" (the possessive that always refers back to the subject of the sentence, which here is 'you') is correct, it's always the better choice. "Dbaj o swoje ciało" is the most natural phrasing, definitely.