User wants to delete a project, but Core does not allow that and says the project is in use. However, there are no time and expense entries or invoices associated with the project. It only has a billing schedule.
This is the expected behavior in Core. When you create a billing schedule for a project, Core creates billing records for the project to be processed as per the schedule. So you cannot delete the project unless you delete the billing schedule first. Check Core Help Center for details on Billing Schedules.