Why does Minecraft say failed to create Java?

Why does Minecraft say failed to create Java? This article refers to Minecraft: Java Edition usernames. When signing up for an account and creating a username you may see this error: IllegalArgumentException: Registration failed. This error occurs when a registered account already exists with the username you have selected.

This article refers to Minecraft: Java Edition usernames. When signing up for an account and creating a username you may see this error: IllegalArgumentException: Registration failed. This error occurs when a registered account already exists with the username you have selected.