This warning occurs when a persistent type mapped to a view has no primary key specified, e.g. when you import views from the database. Telerik Data Access always needs to associate its persistent types with a primary key.
To resolve this warning, select the property you want to be a primary key, and in the Properties pane set Identity to True.
If you prefer to use the Validation Dialog, double-click the warning in the Error List pane. In the Validation Dialog, specify the primary key.