To use the GETPIVOTDATA function, the field you want to query must be a value field in the pivot table, subtotaled at the right level.
In this case, we want the grand total of the "sales" field, so we simply provide the name the field in the first argument, and supply a reference to the pivot table in the second:
The pivot_table reference can be any cell in the pivot table, but by convention we use the upper left cell.
Note: GETPIVOTDATA will return the value field based on current "summarize by" settings (sum, count, average, etc.).