To log out of your Roblox account in Windows 10/11, just navigate to the Roblox home page. Once you’re there, tap on the Settings icon from the upper-right side of the screen then select Logout ...

Usernames will be then be only used to log into your Roblox account. There are a few advantages of display names. First and foremost, unlike the username, the display name doesn't need to be unique.

Ensure that your username is appropriate and meets the requirements. Log into your Roblox account on another device (PC or phone) and attempt to log in again to your Xbox.