I experienced this for an April 2023 Cruise on Regal Princess. Apparently, this is something new that Princess has embraced. I have been sailing solo for over 15 years and was shocked when I tried to book online and it would not allow me so I called Princess and I was told no "single/solo" booking were allowed on that particular cruise, I could book other itineraries without a problem, only the one I truly wanted would not allow. Not sure why they have chosen to do this as we solo travelers pay the base cruise fare for 2. I guess we don't generate enough revenue that is needed to support specific itineraries. I expressed my disappointment with letters/emails to corporate but I sure they went into a 'too bad" file. 😄😄