Characteristic &1 is not assigned to a class at material &2

What causes this issue?

The characteristic is used in dependencies of the current model, but it
is not assigned to a class of the configurable material (including
inherited classes).

System Response

The values for this characteristic are not written to the database.

How to fix this error?

Maintain a class assignment of this characteristic if this is necessary.
This assignment is not necessary for many reference characteristics and
for characteristics which have interim values. But, this assignment is
necessary for reference characteristics to MMCOM-VKOND and SDCOM-VKOND.

Procedure for System Administrators

To find the usages of a characteristic in dependencies, call transaction
CT12; or call the characteristic under Cross-Application Components ->
Classification System -> Master Data -> Characteristics and choose
Environment -> Where-Used List.

Error message extract from SAP system. Copyright SAP SE.