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