This is a problem created by the August 3rd update of the Google App. OK Google and Hey Google detection are broken on many devices and the settings to enable voice in the Google App are greyed out. Further, if you uninstall and attempt to re-install Google Assistant when you attempt to open it a message says your device doesn't meet the minimum requirements of Android 6+. Google is aware of the bug and has a thread on the Google Voice community page. They say they are working on a solution. Several hundred people on that thread have reported identical problems in the past three days. Mine stopped working yesterday and my settings are now greyed out as well. I would uninstall the Google App and replace it with an earlier apk but the Google App is a system app on the eu ROM.