This feature is restricted if the age you registered when you first started the game is underage.

[Notice]
Minors/Underaged players will not have the option to use [Change Player Name]. Several features are restricted based on the date of birth entered upon account creation. It is not possible to change the age of an account once it has been created. After the registered age on the account reaches the legal adult age, these features will become fully accessible. We apologize for any inconvenience caused.