This error occurs when a persistent class is a base class in a horizontal inheritance hierarchy and is mapped to a table in the same time. Horizontal inheritance can only be enabled for the topmost class in a hierarchy. Each immediate subclass is stored in its own table with a "copy" of the fields from the superclass.
To resolve this error, remove the reference to the table for the persistent class:
- Double-click the warning to open the Validation Dialog.
Apply the automatic fix for removing the table.