No active source of supply found for scheduling agreement &1

What causes this issue?

A valid source of supply for the scheduling agreement was not found.

System Response

The system can no longer include the scheduling agreement 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 the existing scheduling
agreement to refer to the new storage location. You can do this with
transaction /SAPAPO/SPP_EP_RO_CP.
If you deactivate or delete sources of supply, you must remove all open
schedule lines of the scheduling agreement and create a new scheduling
agreement release.
You can activate deactivated sources of supply so that you can remove
the open schedule lines and create a release.

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.