& is not a configurable material

What causes this issue?

You have entered a material as a configurable material that has not
been marked as configurable.

System Response

This is not allowed.

How to fix this error?

Check your entry.
The specified material must
either be a material of the material type "Configurable material", or
be a material whose material type is marked as configurable, or
be marked in the material master record as configurable

Error message extract from SAP system. Copyright SAP SE.