You cannot enter a generic and a plant-specific material

What causes this issue?

You have defined a material as plant-specific using a plant number. In
a second definition, you defined it as generic, that is you did not
specify a material number. The system only supports unique assignments.

System Response

The system rejects your entry.

How to fix this error?

Change the material or plant number to resolve the conflict.

Error message extract from SAP system. Copyright SAP SE.