User is trying to delete a project, but cannot delete it. It throws an error:
"...the project is already in use."
User is trying to delete a project that has invoices associated with it. That is why Core does not allow this action. You cannot delete a project if invoices are associated with it. You need to delete those invoices first and any related data, and then delete the project.
The same applies to other records in Core. You need to delete the associated data first and then the main item.