Over the years I have needed to sort a pivot table using a data area
field as the sort key. And over the years I have clicked on a cell in
the data field and gone to hit "Sort" and found it greyed out. So I
wanted to share my solution so that next time I forget how to do it, I
can look it up here!

Imagine a simple phone call report derived from phone company data.
1) Row area has "Called number" in it
2) Data Area has "Sum of Call cost" in it
3) If you click on any cell associated with the Data Area, the call
costs, then there is no option to sort
4) So, click on a Row Area cell - a called number. And from the Pivot
table toolbar select "Sort and Top 10" (or Field Settings/Advanced)
5) The default sort mode is "Automatic", select one of the other
options: Ascending or Descending
6) You can now choose which field to sort on. Select "Sum of Call Cost"

You are now sorted