You are using SmartFill and have created a calculated field. You would like to mark the Search checkbox for the calculated field but the box is unavailable.
Calculated Fields are fields that you define to display information that is not stored in a table field in the database. While these Calculated Fields cannot be displayed in the Make a Selection window, you can use a Calculated Field to provide more information regarding the records you are searching. They may not be used as searchable fields.