You need a FB score of at least 10 in order to do a Fixed Price. I tested one of my Auctions and I DID have the option to change it. And it has to be done thru Ebay's revision page if it's already running. But if you're FB isn't up to 10 yet or you're unverified with Ebay, I don't think you can list at Fixed Price.
This link might help you...