I was going to give it to my Togekiss but it says that it's "Illegal"
The Arceus plates thing is a visual glitch in teambuilder, but Razor Fang really is banned from National Dex. According to their mechanics thread, only Z-crystals, Mega Stones, and items exclusive to particular Pokemon are allowed through. So items like Razor Fang, Custap Berry, etc. are not allowed. If you have issues with this, that would belong in the National Dex forum (probably in the simple questions simple answers thread)
As an aside, you can just use King's Rock, which will be functionally equivalent to Razor Fang
According to that, razor fang was not returned to the game in natdex for whatever reason
Source: https://www.smogon.com/forums/threads/razor-fang-in-gen8-national-dex-ag.3664866/
We've been looking at National Dex's identity as a metagame, and have ultimately decided to alter the current legality rules slightly. Every item that was legal in SM will be legal in National Dex. This means that Custap Berry, as well as Enigma, Jaboca, Micle, and Rowap Berry will be legal for use in National Dex from now.
source
Following the vote outlined in this council minutes post, the council has voted to unanimously ban King's Rock, Razor Fang, and Quick Claw. Bright Powder and Lax Incense remain unbanned.
These items reduce potential counterplay into a luck-based coin toss, which is uncompetitive. These items also disregard any building and playing skill and add nothing relevant to the tier. Bright Powder and Lax Incense remain unbanned because they are far less impactful than the rest of the items voted on. It would also be inconsistent to ban Bright Powder / Lax Incense without touching Snow Cloak and Sand Veil, though we may revisit those abilities / items in the future. Though they are uncommon, it is best to remove the uncompetitive aspects from the tier when they present themselves.
