After completing this exercise, I found it strange that both in my file as the solution file the column "list price" only contained values for the accessoires parent category.
I think this is caused by the value in cell E1 of C04E03 – Accessories.xlsx from the freshly downloaded zipfile on https://www.microsoftpressstore.com/store/collect-combine-and-transform-data-using-power-query-9781509307951
This cell contains a space before the underscore, if you remove it and refresh the query, the column is filled for all categories.