From the Welcome to the Forums! page:
New Forum Usernames
We’re separating your forum user identity from your regular Plex account, giving you the option of having a different username for the forums than what you use elsewhere in Plex. One notable difference with our new forum platform is that it has stricter guidelines for usernames—not allowing spaces, special characters, or email addresses. If your username contains any of those, our new forum will replace them with allowed characters (spaces become underscores), however you can change it without affecting your Plex username. You can review and change your forum username in your Forum Account Preferences .
Therefore… while that name might have been “free” in the forum, it might be taken with regards to the “native” Plex accounts.
