No table bookings

My sons 18th bday coming up next week and he wants to book outside at the new Rocomammas in Rondebosch. Newly opened. I phone to book the entire courtyard to the left for our party which guarantees a few thousand rand in food and drinks and I said im super happy and prepared to pay a deposit. I'm told by staff member and then manager I cannot book. How can I invite all his friends and family to a venue which might not be available. Times are tough, it's new, it's not or never is full outside as I drive past every second day. I don't believe this is acceptable whether it's a head office policy or not. I now won't go there and experience their " extraordinary service".