If you're talking about the website proper: you can't change your username if you've made at least one post on the section you're using. This is because allowing people to change username all the time would cause confusion and drama.
If you're talking about the Discord server: you can't change your nickname if you don't have the Verified role. This makes it so people can't join the server and immediately use a nickname to cause problems.
If you can't change your name and neither of these scenarios apply to you, then you'll need to explain the situation clearly.
