When requesting a reservation online, your customers can only request a time that is covered by the Reservation Sessions that you have created in the backend.
If you cannot see the reservations sessions that you have created:
Go to Reservations > Reservation Configuration
Ensure that all the fields are filled in on the Sessions table: Name, Days of Week, Start Time, End Time, Maximum Guests
Click Sync Data or Save
Note that only future reservation times will display, e.g. if it is 12pm, and you are looking at today's reservations, you will not see any time before 12pm regardless of the reservation session you may have created