Friday, the 13th online special sold out on website

I received an email from KFC marketing to let me know that there is a Friday, the 13th special for the day only of Dunked Crunch Box special where if you buy one for R57. 90 online ten you receive one for free. 16 minutes after receiving the email I tried to order this special online and the KFC website showed that it is sold out. I have screenshots to prove this if needed. This is very bad customer service. Why advertise a special and then it is sold out on the website?