I’m sorry if someone gave you bad feedback. YOU are actually correct! :-) I am a native English speaker, and I can therefore tell you with absolute certainty that you could answer that request/command many different ways. 1) Sure (very casual) 2) Of course. (Causal but much friendlier) 3) Yes “Yes I will” is more of the type of English you would read in a book. It also doesn’t sound terrible friendly. But that last part is just my opinion.

Both are natural.

Both “Sure, I will” and “Yes, I will” are grammatically correct, but they feel different in tone. “Sure, I will” sounds natural, friendly, and relaxed. It is what native speakers often say when they agree to do something. It means “Of course, I’ll do that.” “Yes, I will” sounds more formal or slightly stiff. People might use it in writing or in serious situations, but not much in casual speech. So, “Sure, I will” is preferred here because the situation is simple and friendly.