You need to be a Beta tester of the Digital wellbeing app otherwise you can't install it. You need to sign up on this link: https://goo.gl/tjFBph
Then it will take some time before you can download the app, up to 24 hours. If you can't find the app when you search for it in the play store you can go to the my apps and games menu and scroll over to beta. Then it should be there after you sign up to be a beta tester.
If it doesn't show in settings after installing the app do a restart of the device.