It was not possible to determine a country of departure

What causes this issue?

The system tries to determine the current legal regulations for the
country of departure.

System Response

The country of the delivering plant is specified as the country of
departure. If you have not maintained a plant in the document item, the
system issues a warning.

How to fix this error?

Maintain a plant in the document item, using the appropriate change
transaction.

Error message extract from SAP system. Copyright SAP SE.