člověk is literally "human", "a man" (meaning any sex or gender) or "a man" (a male person in particular)
osoba is literally a person
But the usage of these words differs in our two languages. Where one language prefers one the other language may prefer the other. It is best to see many sentences and observe the actual usage. Most often, both options will be accepted here at Duolingo.
I believe English will more often choose person where Czech will choose člověk. In this particular sentence here "osoba" is also perfectly natural.