Google Voice Assistant is an exclusive feature introduced recently in only a small number of smart phones and Google Pixel Pixel XL, but...
To activate the Google Assistant feature, you will need to go into the system folder and add the following line in build.prop using a file manager :
ro.opa.eligible_device = true
Save the changes,then clear cache, Google Apps data and reboot your device. If you don't see google assistance you will have to you wait a little and restart your device again.