The Allow Nulls property can't be set on column <Columnname> because it is an identity column. (Visual Database Tools)
All identity columns must contain unique values other than NULL. To make the column an identity column, first clear the Allow Nulls check box on the Column Property dialog box.
For more information, see How to: Modify Null Options on Columns (Visual Database Tools).
See Also
Concepts
Error Messages in Visual Database Tools