So, are you saying that when you tap the slider nothing happens?
Have you enabled Google now?
I have been trying to figure out a solution of this problem since a while now. Here's how it works:
1. Go to settings and search for "App permissions".
2. In the list of actions, find "Assist & voice input".
3. Select Assist app as "Google App".
Now on Tap should start working now. Make sure you have enabled it from Google Now settings too.
GLHF
Go to Settings, apps, hit the gear on the top right, Default apps, and set the Assist app to the Google app. Then you will be able to turn on Now On Tap
Go to Settings, apps, hit the gear on the top right, Default apps, and set the Assist app to the Google app. Then you will be able to turn on Now On Tap
Go to Settings, apps, hit the gear on the top right, Default apps, and set the Assist app to the Google app. Then you will be able to turn on Now On Tap