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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).