Character creation can get temporarily closed on any server when it is too busy. This is a temporary state and can change from minute to minute. Try again at some time when the server is less busy - early in the morning is usually a good bet.
This is different from a congested server. You can neither create a new character on a congested server nor transfer a character to a congested server. A congested server will only change status when SE decides it does not need to be congested any longer. Such a change always happens in conjunction with ordinary server maintenance and is accompanied by a notice on the Lodestone and in the launcher.