Only external procurement is defined for the material

What causes this issue?

You have selected the "Co-product" indicator, although only external
procurement is defined for the material. However, it only makes sense
to identify a material as a co-product if it is produced in-house.

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?

Either allow in-house production for the material or cancel the
"Co-product" indicator.

Error message extract from SAP system. Copyright SAP SE.