For some unspecified reason, Mojang has censored the name u40 from being used, even though no profiles use this name.
If you look this name up on other websites, sometimes it will have no users, and other times there will be a UUID associated with it. Either way, however, you won't be able to change your name to this.
Mojang doesn't provide any reasoning or clues as to why this happens. Most of the time, it's because it's a vulgar name, however sometimes there is no obvious reason.
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