"Are you afraid of this person?"
Translation:Téhle osoby se bojíte?
"Se bojíš této osoby" was not accepted because of the placement of the word se, not because you used second person singular.
Se has to be the second word or the second "unit of meaning" in the sentence or clause. In the correct answer that you were shown, it is the second word in the sentence. In the main translation shown at the top of the page ("Téhle osoby se bojíte"), it is the second unit of meaning.