SUMPRODUCT is not exposed to Worksheetfunction, you have to evaluate itt

sFormula = "SumProduct(--(A2:A" & LastRow & "=" & Cells(1, C).Value & _
                     "),--(C2:C" & LastRow & "=""2005/02""),(F2:F" & LastRow & ")"
            Cells(X, C).Value = ActiveSheet.Evaluate(sFormula)