Can't enchant tridents

I've been waiting to enchant my tridents for a while now, and now that I've got all the supplies (exp, table, and lapis) it won't let me enchant them. (I'm able to enchant all the other stuff as usual). I'm playing on the PS4.

Enchanting a Trident requires several things

• An unenchanted Trident
• Lapis (1-3)
• Experience levels (at least 8)

Sometimes, you might get a Trident that has an existing enchantment on it already, which means that you can't enchant it further. In this situation, you can instead combine it in an anvil with either another enchanted Trident, or an enchanted book.