You have a field named "County" and you set the criteria to "County"?
To answer your question, don't use the name of an actual field. Instead of [County], use something like [MyCounty], [xCounty] or [EnterCounty].
But I would never let users run a query and be able to edit/change the data in the query. That is what forms are for.
I would have an unbound combo box in the header of a form with the row source set to
Code:
"SELECT DISTINCT County FROM TableWithCountyField"
In the after update event of the combo box I would have "Me.Requery"
The form recordsource would be the query you now have. The criteria for the "County" field would be the combo box name instead of "[County]".