I am having the same issue on my iMac (Retina 5K, 27-inch, Late 2015). Wi-Fi icon in the menu bar has a line through the Wi-Fi icon. When I click on the Wi-Fi icon from the menu bar, it first beachballs, then the drop down appears, and then I can attempt to turn Wi-Fi on. However, it will not turn on.
Going into System Preferences > Network > Wi-Fi, at first the settings under Status: says "<<From Localizabl..." after a few second that disappears and then the option to "Turn on Wi-Fi" is there. When clicking on "Turn on Wi-Fi" it does nothing.