As you can see, “No matches found” is still appearing instead of the custom field, even with “show custom field on printed documents” being checked.
I did search the guides and forum but the answer was not clear. “Check Show custom field on printed documents to display the field on forms” does not explicitly explain that it needs to be checked in order to appear as an option on report.
@ZzBlueBird42 to get quantity column, use Sales Invoices - Lines screen to create your custom report. To access Sales Invoices - Lines, go to Sales Invoices, then click Sales Invoices - Lines button in bottom-right corner.