Hello,
PPM and at least one resource must exist in the destination location
The PPM must also be active.
Product specific transportation lane between plant and vendor location should be there.
Refer SAP help link,
Subcontracting Without Source Location in PP/DS - External Procurement - SAP Library
And FAQ SAP note 1905822 - FAQ: Subcontracting in PPDS
Best Regards,
R.Brahmankar