This error means that the table with name SubCustomer is not available in the database.
As you are using vertical inheritance strategy, this means that each class is mapped to separate table. So OpenAccess assumes that your class SubCustomer is mapped to a table named SubCustomer. However, this table is not present in your database by default, so you need to create it by running the Update Database From Model wizard
. This will add the needed tables to your database and should fix the error at runtime.
the Telerik team
Q3'12 SP1 of OpenAccess ORM packs Multi-Table Entities mapping support. Check it out.