Barcelona was the cruise port I was most looking forward to last August 2023. It was very crowded and hot.
Now I do not know why the NCL excursions are not showing up, but I would recommend a private tour. (or at least a semi-private tour)
We used Pepito tours and Jose Sole is the tour guide who responded to us and made arrangements. (He might be the owner) We had a private guide (not Jose, but a knowledgable, fun young woman from Barcelona) and driver. We were able to zip around and beat the crowds. Jose has one of the tours recommended by Rick Steves. We did all things Gaudi, but I think you can customize their tours. They met us at the ship.
It was fantastic. Our guide even recommended a fabulous tapas restaurant for lunch when we were finished. There was a line waiting to get in and she helped us skip the line and get a great table. We met our son and DIL there for lunch. It was my favorite day on the cruise. I would love to go back there someday. Good luck!