It's greyed out in the weapon wheel. I've not been able to use it as a weapon for the past few sequences. If I replay the level where you unlock it, the slave persona has it, but the assassin's persona doesn't and even then, you can't use the predator move with it while in the slave persona. As soon as you swap it out for a different weapon in the slave persona, it disappears completely again. A few people seem to have come across this glitch, but as far as I'd seen on other forums, there's never been a way to correct the issue. So I'm going to have to play through a new game just to unlock the whip for that trophy. It's annoying, but well, for a platinum, I guess worse trophies could have gotten a glitch.