This is true for Barefoot Beach which is for suite guests and pinnacle members (gold sea pass passengers). If not taken by them, D+ are next in line to book through the CL. Suites are booked through the concierge. However Nellie's Beach also has cabanas that anyone may book. There are not too many and they go very quickly. They can be booked online, so my guess is the allotted number for Nellie's either are already booked or for the OP's sailing they have not been offered yet.