A primary key can't be created on column <Columnname> because it allows null values. (Visual Database Tools)
The primary key is used to relate the table to foreign keys in other tables. All columns participating in a primary key must contain defined values other than NULL.
To create a primary key on the selected column, first clear the Allow Nulls check box for the column.
For more information, see PRIMARY KEY Constraints and How to: Modify Null Options on Columns (Visual Database Tools).