Sending a range to function

Private Function FormulaFiller(idata As String, istart As String, iend As String)
    
    Dim rgCopy As Range, rgDelete As Range, rgFill As Range, rgClear as range
    
    Set rgCopy = ["istart"&:&"iend"]    'Formulas in these cells will be copied down
    Set rgFill = ["idata"]       'First piece of raw data

    set rgClear = range( ' need to clear the data/formula below istart and iend 
    Set rgFill = Range(rgFill, rgFill.Worksheet.Cells(Rows.Count, rgFill.Column).End(xlUp))  'All data in that column
    Set rgFill = rgCopy.Resize(rgFill.Rows.Count)       'Fill formulas down in this range
    
    Set rgDelete = rgCopy.Offset(1, 0).Resize(rgCopy.Worksheet.UsedRange.Rows.Count)
    rgDelete.ClearContents
    
    rgCopy.AutoFill rgFill, xlFillCopy
    Application.CutCopyMode = False

End Function

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Hi,

I have the about function. However, I am getting an object not found error on these lines

    Set rgCopy = ["istart"&:&"iend"]    'Formulas in these cells will be copied down
    Set rgFill = ["idata"]       'First piece of raw data

    set rgClear = range( ' need to clear the data/formula below istart and iend

As you can see, I am getting a range from different function and trying to 'pull down' formulas. But I am having hard time bringing in ranges as string :/
If you see this thread, you will understand: https://www.experts-exchange.com/questions/27383431/Extend-formulas-using-macro.html

I also need to clear the space below the initial formulas before those fields get populated. I started on a line but my solutions are not working/giving errors.

Any help is much appreciated!
Thanks!