INDEX formula not working

Hello,

may I ask you for your help with following code? I have been struggling for some hours and now I seem to be lost completely.

What I am trying to do: I have a table with some document details starting the area from A6 to K6 and with variable count of rows. I need to choose randomly 20% of them and put an "X" in the column L. My approach is following:

Code:

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'In the column L, put a formula which will read the randomly chosen document numbers (from column P) and put an "X" when found
    With Sheets("Sheet3")
        lngLastRow = .Cells(.Rows.Count, 2).End(xlUp).Row
        .Range("L6:L" & lngLastRow).FormulaR1C1 = "=IFERROR(IF(VLOOKUP(RC[-7],C[4],1,0)=RC[-7],""X"",""""),"""")"

   
    'Copy the document numbers from the column E into the available column N
        .Columns("E:E").Select
        Selection.Copy Destination:=Columns("N:N")
       

    'Use RAND formula and paste it as a value
        .Range("O6:O" & lngLastRow).FormulaR1C1 = "=RAND()"
        .Columns("O:O").Select
        Selection.Copy
        Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks:=False, Transpose:=False
        Application.CutCopyMode = False
    End With
   

    'Use Index formula to determine the random values
    With Sheets("Sheet3")
        lngLastRow = .Cells(.Rows.Count, 2).End(xlUp).Row
        .Range("P6").Select
        .Range("P6:P" & lngLastRow).FormulaR1C1 = "=INDEX(R6C14:R & lngLastRow,RANK(RC[-1],R6C15:R & lngLastRow))"
        End With

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I always get stuck on the last line:

Code:

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.Range("P6:P" & lngLastRow).FormulaR1C1 = "=INDEX(R6C14:R & lngLastRow,RANK(RC[-1],R6C15:R & lngLastRow))"

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I really don't know what is wrong. Can you please help me?
Thank you very much, SaburaZera

Try

Code:

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.Range("P6:P" & lngLastRow).FormulaR1C1 = "=INDEX(R6C14:R" & lngLastRow & "C14,RANK(RC[-1],R6C15:R" & lngLastRow & "C15,0))"

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