No valid identifier for reference material (SUBID: &1)

What causes this issue?

Cannot uniquely identify the reference material used in this product
structure.

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?

Release the material separately to obtain a unique node ID.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.