I have created my event and added the ticket but the ticket is not shown on the checkout page, why?

Every event has a Start and End date/Time and also every ticket sale has start and End date/time. If the ticket sale start date is of future date than the current date/time then the tickets will not show up.  In order to make your tickets visible, please set the Ticket sale start date/Time to current date/Time.   

Also, check the status of your ticket whether it is in the Published state or in  Pause state.    If out of many tickets, you want to have one or some of your ticket to be available for sale then just set the ticket sale Start date/Time to a future date/Time, don't change the status to Pause. If a ticket is  Paused, it will never be available for buyers.