What causes this issue?

A valid source of supply for the purchase requisition was not found.

System Response

The system can no longer include the purchase requisition in planning.
A valid source of supply consists of:
a valid external procurement relationship,
a valid product-specific transportation lane which the external
procurement relationship is assigned to,
a valid means of transport which can be used on the product-specific
lane.

How to fix this error?

If you change the storage location of a source of supply (plant to
contract packager, contract packager to plant or contract packager 1 to
contract packager 2), you must reorganize existing purchase requisitions
to refer to the new storage location. You can do this with transcation
/SAPAPO/SPP_EP_RO_CP.
If you deactivate or delete sources of supply, you must remove all open
schedule lines of the purchase requisition.
You can activate deactivated sources of supply so that you can remove
the open schedule lines.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.