Hi rockingrebels,
Sometimes it isn’t possible for Events Calendar PRO to communicate with the key validation server due to constraints within the hosting environment. It’s also possible of course that there was a temporary glitch on our side.
Certainly do try again (you could also try doing so without any other plugins activated, and only a default theme, to check in case something else is interfering) and hopefully it will work as expected.
With that said, if you continue to hit problems, do note that not having a license key will not prevent you from using Events Calendar PRO: all of its features will function as expected. Operationally the only side effect of not being able to register a key is that you will not be able to take advantage of automated updates and you can take care of this with manual updates.
Does that help at all?