Did you buy it knowing it ate live of frozen? You might one day switch to frozen but you also might never get that to happen.
Most likely the location that sold you this, if they did not disclose the feeding history, are just flippers and sold you a snake without regard to your feeding preference.
No amount of husbandry can change a snake to frozen thawed. They either do it or they don't. If they do not it is nothing you did aside from buying it without getting the feed history.
Feed it what it will take. You can try every few feedings to switch but be prepared to toss the prey as you experiment. Don't refreeze warmed frozen prey.