Order &1: No main material defined

What causes this issue?

If you created the order in the R/3 System:
No main product exists in the BOM, that is, no item one of the BOM
exists.
If you created the order in APO:
No main product exists for the PPM.

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?

If you created the order in the R/3 System:
Define a main product by entering an item one in the BOM.
If you created the order in APO:
This is a program error. Inform your system administrator.

Error message extract from SAP system. Copyright SAP SE.