Thank you for replying with that suggestion, I've tried before and just tried again, no luck unfortunately. Checked With all kinds of devices and browsers, cleared all cache, cookies etc.
It doesn't seem to be an issue just for me, e've booked it in advance for previous cruises, and clearly it's available for some cruises as people mention above.
My best guess is either a data fill issue for specific cruises or an intentional block on it (for some unknown reason).