Problems running a dedupe macro...stumped

Option Explicit

Sub Column_Sort_DeleteDuplicates_()
    '   Code by 
    Dim col%, c%, x%, rng As Range
    col = 4  '1 is the column number to search - change it as required
    '   I.E. Column A = 1, Column B = 2, etc

    '   Set the column number of the column to store temporary formula
    c = Range([G1], ActiveSheet.UsedRange).Columns.Count + 1
    '   Exit if the used columns=256 (instead of exiting, could amend thecode _to look for a column containing no data to use for the temporaryformula)
    If c > 256 Then Exit Sub
    '   Set the column offset(from the search column to the temp. formulacolumn)
    x = col - c

    '   Turn off screen-updating and set calculation to manual _
    (if required, could add a test to check if calculation is already set _
    to manual, in which case would not want to set to automatic at the end
    With Application
        .ScreenUpdating = False
        .Calculation = xlCalculationManual
    '   Set the range to contain the temp. formula
        Set rng = Range(Cells(1, c), Cells(65536, col).End(xlUp).Offset(, -x))
        With rng
    '   Sort the data by the search column
            .EntireRow.Sort Key1:=rng(1, x + 1), Order1:=xlAscending, Header:=xlNo
    '   Put temp. value in the fors cell of the temp.column
            rng(1) = 1
    '   Enter the temp formula that identifies duplicates
            .Offset(1).FormulaR1C1 = "=IF(RC[" & x & "]=R[-1]C[" & x & "],"""",1)"
    '   Convert the formula to value
            .Offset(1) = .Offset(1).Value
    '   Sort entire rows by the temp. formula result so as to group all of_the rows to be deleted at the end
            .EntireRow.Sort Key1:=rng(1), Order1:=xlAscending, Header:=xlNo
    '   Error handler in case there are no duplicates
           On Error Resume Next
    '   Delete the duplicate rows
            .SpecialCells(xlCellTypeBlanks).EntireRow.Delete
    '   Reset error handling
            On Error GoTo 0
        End With
    '   Delete the temp. formula column
        Columns(c).Delete
    '   Reset the sheet's last used cell
        ActiveSheet.UsedRange
    '   Turn on screen-updating and set calculation to automatic
        .ScreenUpdating = False
        .Calculation = xlCalculationAutomatic
    End With

End Sub