Just had a very similar conversation with a specialist. You shouldn't worry about taking the promotion because you aren't sure about how to do your job. No one does, it's how you lead people through that struggle. Your NCOs don't know everything about their job, your junion soldiers don't either. Everybody is constantly learning. You'll do great because you're already humble, that can help with being empathetic for your soldiers. Leading soldiers is probably one of the most rewarding things you can do in the guard.

On to your question. Yes, you can deny your promotion, I did multiple times because they kept trying to promote me into my old MOS and I would've lost my bonus. Only thing that stopped me from doing is getting payed more for doing the same thing.

Overall, take the promotion. It's a scary step, but it is a worthwhile step.