Can’t Assign Abbreviation

I am trying to assign an abbreviation to a spreadsheet. I own a MacPro and the spreadsheet file is on my 2nd Data Drive/Hard Disc. I am receiving the following error message:

Can’t Assign Abbreviation

The selected item is not contained in your LaunchBar index, therefore it can't be selected via abbreviation search.

I did not have this issue in Butler. Is there something that I need to do with LaunchBar that I am unaware of?

Thank you.

Bruce