Welcome! This forum has a treasure trove of great info – Scouters helping Scouters! Just a heads up, though - all content, information, and opinions shared on this forum are those of the author, not the BSA.
Since you son is not in a pack in Scoutbook, did you use the report where you entered his BSA Member ID and last name or from his reports page? This will help us narrow down the problem.
And I assume you mean the Cub Scout History Report. There is no report named Cub Scout Progress Report in Scoutbook.
As a workaround, you can try going to My Dashboard → Reports → Cub Scout History Report then entering his BSA Member ID and last name. You can get his BSA Member ID from his Edit Profile page.
Thanks. FYI - I did just manually add him to his troop of choice. They have the paperwork filed at the Council, but it’s not all updated yet. Anyway, the result is that the error is gone and I was able to print his Cub Scout History Report. So the flaw seems related to not being in an assigned troop or pack when trying to access the report.