Personally, I think it's due to not having Flying Blue status. Do you have status with Delta or another Skyteam airline or flying in Business? This was discussed recently in another topic. You should be able to reserve on arrival in the lounge but the sleeping cabins are first come first serve. If you want you can book a room at the airside hotel (Mercure) or just go landside and go to the Sheraton/Hilton/CitizenM.