Registration was not processed in a timely manner.
From what I understand, you didn’t sign up early enough. The limit was Monday, October 19, 2015, 01:00 AM. That means it was actually finished on “the night” of October 18th… October 19th 1AM being the night between the 18th-19th, or in other words the begging of the 19th, and not the end of the day of the 19th. A lot of people got caught by this thinking that they had the whole day of the 19th to register, which was false.
Also you have to take in to consideration that they use EDT and EST time. If you do not live in that time zone, that might have affected the sign up as well. Here is the current EDT time for more information : http://www.timeanddate.com/time/zones/edt
Finally you might have clicked the button past the time limit. Since the button was still present to click after the limit, there have been many people who clicked it thinking they would get a key even past the limit. And I think that is what happend and why you got the e-mail.