I really like this course because of its high quality in accepting translations. Many other courses would not have accepted the grammatically correct, "Of what is he scared?" but this one did.
"Av" isn't used in this kind of situation. It'd take too much time to explain it, I can only say you'll eventually learn it by giving yourself some time, reading stuff, listening to people, etcetera.
I imagine not as the 'fear' in the English sentence may well be idiomatic rather than literal, so it would make more sense to apologise instead of refer to fear, as you may well end up implying that you are SCARED of coming rather than unable to.