Either carry out supply source determination or select supplier

What causes this issue?

You selected automatic supply source determination and manually entered
the source of supply.

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?

Use an alternative.
If you want to enter the source of supply manually, you must enter a
vendor or a plant.

Error message extract from SAP system. Copyright SAP SE.