Customer & not maintained

What causes this issue?

A customer that has not been created in the system, or has already been
archived, is assigned to the plant.

System Response

The plant data cannot be changed. If the customer has already been
archived and deleted, changes to the address can lead to
inconsistencies. The site data can only be displayed.

How to fix this error?

Check whether the customer has been archived and deleted. If it has
been, you can retrieve the customer from the archive.

Error message extract from SAP system. Copyright SAP SE.