Material & only listed as component in assortment &

What causes this issue?

A material can only be ordered if it has been listed as a single
material or as both a single material and a component of a structured
material.
If you want to order a material that has only been listed as a
component, you have to order the whole structured material.

System Response

The system will reject this purchase order.

How to fix this error?

Order the whole structured material.

Error message extract from SAP system. Copyright SAP SE.