This error usually occurs in model first scenario when a nullable column from the relational part of the model is specified as a primary key.
To resolve this error, change the nullability of the column. You can do this by using the Table Editor:
- Right-click the entity in Visual Designer and select Edit Table... from the context menu.
- In the Table Editor find the primary key column and change the Allow Nulls setting.
If you prefer to use the Validation Dialog, double-click the error in the Error List pane. In the Validation Dialog, change the nullability of the column.