Repeating sessions BookingNinja works by setting a time slot, and a booking duration for the session, and then setting an open and close time - you can only set specific time slots for events.
This is because it would take most venues a very long time to set up each slot individually, and be very difficult to edit hundreds of time slots when wanted to make changes.
While you can’t set specific slots, this way, you can still have variation within the same session by setting multiple open and close times on the same session.
For example, you run usually run a time slot every 60 minutes, so have a 60 minute interval, but you take a 90 or 120 minute lunch in the middle of the day.
This process is described in full here.