To create a project, you must set your create scope to the plant level. The SPFProject class definition instantiates the ISPFClassDefExt interface, which has a property SPFDependantConfiguration value of SPFPlant. When objects have a value for the SPFDependantConfiguration property, they can only be created at that level. In this case, projects can only be created when the scope is set to the plant level, not if the scope is set to Configuration Top or a project.