Here is what I found: open the file BookReportTemplate in FrameMaker and save it.
Then run the script again (no need to restart as the script is still linked via the menu).
Apparently, the SimpleNewDoc command does expect the file to be in the FM version you are working in, and the BookReportTemplate is likely to be an older FM version, so opening the template as a new document failed.
You nailed it! Once again, thank you kindly, Heer Jang.