When you create a new service under Configure, you need to set the duration under Time Settings. Under Advance Time Settings, you will be able to set the booking time step - this must be less than the duration. E.g. your service can have a duration of 1hr and you can set the booking time step to be 15mins, this means that the service can be booked four times in that hour.
Please note: To enable a booking time step, the service type for your business must be fixed time or variable flex time (Account - Settings - Service Settings).