Purch org. & is not valid for exchange partner &

What causes this issue?

You have to enter a purchasing organization for which the vendor master
data is maintained.

System Response

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

How to fix this error?

Check the purchasing organization you entered.
Maintain the purchasing organization in the vendor master data.

Error message extract from SAP system. Copyright SAP SE.