Google Assistant is available for phones running Android 6.0 and later. Before you can say "OK Google" to launch Google Assistant, you need to set it up and teach it to recognize your voice. To set up Google Assistant, connect your phone to the Internet and then press and hold /Fingerprint scanner.
For Google Assistant to respond to "OK Google" and voice commands, Power saver in Settings must be turned off.
To check if Power saver is turned off: