You already have the maximum amount of characters on this account

[Mizore-icecrown]

@Kaivax

Today 1:00 PM , we will remove the limitation on only creating a total of three WoW Classic characters per WoW account. Thereafter, players will be able to create up to 10 characters per Classic realm, for a total of 50 characters across all Classic realms per WoW account.

They blew it.