Sorry, I (maybe) found the problem. The book omitted the instruction to use the "swift repl" command before the print command. Once I did so, the print command worked properly.

Thanks Claude31. I'm just following the instructions in the book mentioned in the post. I found the problem: those instructions don't specify running "swift repl"; they just say to run swift and then run the print command. Although I'm just starting to update my programming skills, this taught me a lesson: documentation is often misleading!