When you specify the Inheritance strategy to be 'Vertical' this implies that each class has it's own physical table. Hence you get the error that 'Employee' table is missing.
If you want to store instances of both Person and Employee in a single table 'Person', you need to use the default Inheritance strategy, which is 'Flat'.
In case you want to create the additional 'Employee' table then you can use the Schema Handler API and let OpenAccess generate the apporpriate ddl.
var context = new EntityDiagram();
var schemaHandler = context.GetSchemaHandler();
var ddl = schemaHandler.CreateUpdateDDLScript(null);
if(string.IsNullOrEmpty(ddl) == false)
schemaHandler.ExecuteDDLScript(ddl);