An index can't exist on a blank column. (Visual Database Tools)

An index must contain at least one column, and that column must contain unique data values if you are creating a unique index. The column names selected must be entered under Column name in the Selected index box.

For more information, see Creating Indexes (Database Engine) and How to: Create Indexes (Visual Database Tools).

See Also

Concepts

Error Messages in Visual Database Tools

Help and Information

Getting SQL Server 2005 Assistance