AFAIK it's because exp_Gross Sales isn't known until the query runs. I'd try either just entering a value or just hit OK rather than Cancel (that's an assumption) or edit it in sql view and save.
As soon as I click Ascending or Descending in design view, I get prompted
EDIT - out of curiosity I tried to replicate your issue and cannot. I only get a prompt when I run it, not in design view. If you're not saying that, then I think you will have to use a function to return a value to the query. Not sure if that will solve it or not.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.