Can't send a message from Contacts - greyed out send button
The "Contacts" application allows the user to click on the word "mobile", next to a contact's mobile phone number, then select "Send Message".
When I do that a window appears, where I can type in a message (which I assume is the same thing as a 'text message', to send to that person's cell phone).
However, the "Send" button is greyed out and inactive. I can't figure out what I'm doing wrong, or how to activate the "Send" button.
Please help.
Thanks!
MacBook Pro (Retina, 15-inch, Mid 2014), OS X Mavericks (10.9.5)