I've had this on my Google Play version Nexus 4 since Android 4.4 and its two subsequent updates. I've been in touch with Google tech support for the past two weeks and still no solution. I wiped the cache from recovery mode and had "OK Google" appear intermittently for a couple of days, afterwhich it returned to the same screen shot shown above. All my settings are perfect, and there's no message appearing like the one you'll see if you go to Google Now menu, to settings, to help and feedback which displays the following: