If you get an error saying, "error occurred during authentication" via iMessage or one saying, "error occurred during activation" via FaceTime from you Mac, please call into apple support, from the legit apple website. This happened to me and everybody kept saying "check time zone" and "log out". None of that stuff worked for me. I called in and found out that something sent from my MacBook may have went to spam on someone else's device and because of that, my Mac device was blacklisted from the server. A supervisor removed it for me and I was able to log in just fine. Also, its free to contact apple support.