DFF for existing and future Assets

I have a business requirement to add the county the asset is located in to a dff in fixed assets.  I set up the dff as shown in the attached screen prints.  When I try to add it on the assignments screen to an existing asset I get the message that at least one assignment must be adjusted before saving changes.  Also, when I click on the 'books' button on the asset workbench, enter the book and hit tab, it requires me to enter the dff but doesn't save it.  Do I have something wrong in my setup?  Should I