This will be because the dates set up for the ticket sales do not include today's date - but don't worry, this is very easy to fix.
If you go to:
- My events in the top right corner of the screen
- Edit next to your event
- Advanced Settings cogwheel symbol (found to the right of your ticket types)
You will then see the start and end date that has been set up for your ticket sales. Amend these dates as appropriate and hit Publish to update the changes.