Can I run a report for tracking classes in bills, using the "Category Details" and not the Item Details?

Hello and welcome to the Community, carmine-marotta-

The purchase by Class Detail Report only draws information from the Item Detail section when creating a Bill or Expense. For this reason, Bills/Expenses that only have data entered in the Category Details table will not post to the report. 

This is working as designed, and the suggestion would be to edit Bills/Expenses to use products/services instead of direct expense accounts, or use a different report (such as the Profit and Loss by Class) to see a break down of expenses by class.