If you don't get any options, it may be because the selected campsite is fully booked or because the campsite only accepts weekly bookings. Contact the campsite directly if you don´t get any options.
If you do not get any options, the selected campsite can be fully booked. It may also be due to the fact that the campsite does not have its entire accomodation available online or that they only accepts weekly bookings. It is also common with various restrictions, especially during the high season or major holidays.
Try to contact the campsite directly to book your accommodation.