Updates on Character Creation Restrictions can be found here.
During periods of high congestion, we may institute a temporary restriction on the creation of new characters.
This information will be updated at the following times:
1:00 / 6:00 / 11:00 / 15:00 / 20:00 (GMT)
2:00 / 7:00 / 12:00 / 16:00 / 21:00 (BST)

○：Character Creation Possible ×：Character Creation Restricted

× Gilgamesh

When you open the client, you can see this information on the left, it was updated 25/04/2017 01:00 AM


Personally, I recommend you get the character made, data saved and a name picked out. Wait till just before maintenance, or just as servers come back up form a maintenance and then try to make the character.