Turns out, Apple's App Store can't accept the new name for Twitter's iOS app because of minimum character requirements.

https://www.bleepingcomputer.com/news/technology/apple-rejects-new-name-x-for-twitter-ios-app-because-rules/

Apple rejects new name 'X' for Twitter iOS app because... rules

Mr. Musk may have successfully pushed Twitter's new name and logo, 'X', and even made the vanity domain x.com redirect to the social media website, but that's not to say, the Mathematical double-struck letter will fit the bill everywhere. Apple's App Store can't accept Twitter iOS app's new name due to minimum character requirements

BleepingComputer
@bruno_j_navarro @kcarruthers Vectornator has been renamed to “Linearity Curve (Vectornator)” not sure which is worse.

@bruno_j_navarro Elon & the 3 developers who still show up to work figured out a temporary way around this problem by renaming the app just for Apple's requirements "X The App Yes That One Nothing To Do With Porn"

According to internal reports, things are going even better now.

@bruno_j_navarro
I am wondering if he will be able to trademark a letter. Does that even work?