What causes this issue?

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

System Response

The system can no longer include the purchase order 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 deactivate or delete sources of supply, you must remove all open
schedule lines of the purchase order and release the purchase order.
You can activate deactivated sources of supply so that you can remove
the open schedule lines and release the purchase order.

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.