The servers are full, ergo, no new characters can be made.
Space for character data is not reserved when you purchase the account; space is only used when you create the character. From the server perspective it doesn't matter if its a second character on an existing account or a new character on a new account, character data is character data. If the server is full, there's no room for a new character.
As others have said, when servers are not full. Monday is maintenance to try and increase the server capacity, it may free up enough space to open Siren for new characters or it may just ease instabilities. We won't know what will be available until they bring the servers back online and tell us what the capacity is.
It was the correct answer though. Nobody besides the devs can give you any specific timeframe for more information and chances are even they don't know.