The Identity property can't be set on column <Columnname> because it allows null values. (Visual Database Tools)
Identity columns contain system-generated sequential values that uniquely identify each row within the table. Therefore, identity columns cannot have null values.
For more information, see How to: Modify Column Identity Properties (Visual Database Tools) and How to: Modify Null Options on Columns (Visual Database Tools).
See Also
Concepts
Error Messages in Visual Database Tools