The option to create "Push" notification channels is not available on the UI as incorrectly mentioned in the documentation pages:

https://docs.servicenow.com/bundle/london-servicenow-platform/page/administer/notification/task/create-channel.html

1. Login into an instance.
2. Click on Settings and then on Notifications.
3. Under Notification Channels, click on Create Channel.
4. Select the type. Notice there is no 'Push' choice available.

This is expected behaviour and the docs pages need to be corrected. Users do not manually create push devices, these are automatically created when a user logs in using the mobile application.