On the other hand, if you'd like to run and manage transaction reports by class, I suggest accessing the Transaction Detail by Account report and checking the Class box from the Filter settings. I'm gonna show you how:
Click on the Report menu.
Look for the Transaction Detail by Account from the search field then open it.
Enter all necessary information.
From the report, select the Gear icon.
Put a checkmark for Class.
Refresh the page to display class on the report.
For more information on how you can handle class tracking in QBO, read these: