Can't set ControlSource to fields that don't exist.
If you want "placeholders" columns then need to specify names in the CROSSTAB PIVOT clause with IN().
PIVOT [Stock Location] & " (" & [Column] & ")" IN("Main Office (Qty)", "The Hub (Qty)", "Airport (Qty)", "Agent - Laura (Qty)", "Agent - Johnny (Qty)", "Main Office (Volume)", "The Hub (Volume)", "Airport (Volume)", "Agent - Laura (Volume)", "Agent - Johnny (Volume)")
Again, look at Allen Browne's article for more info.