Up until now when I clicked on the Messages icon it would, of course, open up Messages. Today, when I do that a window comes up asking me to sign in with my Apple ID to activate iMessage.
What the heck is iMessage? When I click on the link "Learn more about iMessage" it takes me to Apple's website and it says "The page you're looking for can't be found."
MacBook
Pro, OS X El Capitan (10.11.6)
