There are a few different reasons why your event might appear on your public page, but say "Not On Sale". This article will walk you through those different scenarios.
The first place to look will be in your Event Settings. Under the Event Status on the right, there dropdown to configure whether your event registration is open or closed. If it is closed, it means that sales are not enabled. If you want attendees to be able to sign up for the event, change this to "Open".
Another reason your event might appear as Not On Sale, is if the event has reached maximum capacity. You can check the capacity limit that you have setup on the Event Settings Tab.
Public/Private Attendee Types
If sales are enabled and you have not reached maximum capacity but your event is still showing as Not On Sale, you will want to make sure at least one of your Attendee Types is set to Public. If all of your Attendee Types are set to private, your event will not be for sale on the public page.