Timesheet entries must be categorised by client, project and task. TimeSite Pro doesn't permit timesheet records to be created without such a categorisation. It is also possible to include a Sub-Project or Project Item in this timesheet record categorisation: client, project, sub-project and task.
Enable Sub Project setting
Go to Settings > System Settings > Features and click on Enable Sub Projects or Project Items in the Sub Project section.
Create a Sub Project within a Project
Go to Admin > Project > [select a project] > Project Items and select Create new item. Add a new sub-project item.
You will then be able to select a sub-project when you select the project in the Add Timesheet Record panel.