"Please relax. What are you afraid of?"
Indeed. Sounds more nature. I'm native speaker, but the origin sentence confuses me. I never hear people say like that...cause behind 請放心 shouldn't be the question. It's always affirmative. For example, 請放心，我會處理 Don't worry, I'll take care of it.
"你害怕什麼" has 3 meanings. 1.What are you afraid of. 2.Why are you scared. 3.Don't be scared. In this sample, as native speaker, I think 2,3 are acceptable, but 3 is proper. 1 is incorrect certainly.