Index Match Help!

I'm trying to use Index Match formula instead of Vlookup due large amount of datas .
Ive heard that its faster than using Vlookup formula .
However , I encounter an error along this line .

Range("S2").FormulaR1C1 = _
    "= INDEX([PatientMerge.xls]2015!C10,Match(C[-16],[PatientMerge.xls]2015!,C10,0))"

I dont know what went wrong with my formula
Please help

Private Sub Unsuccessful()
'Update Column S and T
'S = Active Ext ID , T = Inactive Ext ID

Dim MaxRowNum As Long

With Application
        .ScreenUpdating = False
        .Calculation = xlManual
End With

Sheets("Simpat").Select

'Find the max number of row
MaxRowNum = 1
        
    Do While Cells(MaxRowNum, 2) <> "" Or Cells(MaxRowNum + 1, 2) <> ""
        MaxRowNum = MaxRowNum + 1
    Loop

Range("S2").FormulaR1C1 = _
    "= INDEX([PatientMerge.xls]2015!C10,Match(C[-16],[PatientMerge.xls]2015!,C10,0))"

With Application
        .ScreenUpdating = True
        .Calculation = xlCalculationAutomatic
End With

End Sub

maybe use just .formula, not .formulaR1C1

Thank you so much guys! Its working now.