The bluepost about name reservations says “After Season of Mastery begins on 16 November, the limits of one character per WoW account and the maximum number of characters allowed per realm will be removed.”
The limits are currently still in place and if you try to make a new character you get an error message “You already have the maximum number of characters allowed on this account.”
I’m able to create more than one character per account. You have probably reached to total limit of 50 characters which is shared between classic era and season of mastery. That limit won’t be removed.