Material &1 cannot be expanded

What causes this issue?

Material &V1& is neither a generic material nor a structured material,
therefore it cannot be expanded.

System Response

Planning at the lowest level is not possible for such materials.

How to fix this error?

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

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.