Fourier analysis with variable input

No sample workbook provided so try (can't test):

Application.Run "ATPVBAEN.XLAM!Fourier", Worksheets("Red").Range(Cells(1, i).Address).Resize(rowsize:=128), _
 Worksheets("Analysis - Red").Range(Cells(430, 48 + i).Address), False, False

Notice that "$A1:A128" and "AV430" in the first are string references which are required by RANGE(). Cells(r,c).Address returns the string that Range is expecting.

Hi freirema and welcome to the forum,

As per forum rules, please use code tags around your code.

to answer your questions, make sure you reference the sheet correctly when you use the Cells(x,y) otherwise you may refer to the incorrect sheet (activesheet will be used)
Also when defining a range, you need to specify 2 cells.
For example :

Application.Run "ATPVBAEN.XLAM!Fourier", Worksheets("Red").Range(Worksheets("Red").Cells(1, i), Worksheets("Red").Cells(128, i)), _
Worksheets("Analysis - Red").Range(Worksheets("Analysis - Red").Cells(430, 48 + i),Worksheets("Analysis - Red").Cells(430, 48 + i) ), False, False

or

Application.Run "ATPVBAEN.XLAM!Fourier", Worksheets("Red").Range(Worksheets("Red").Cells(1, i), Worksheets("Red").Cells(128, i)), _
Worksheets("Analysis - Red").Cells(430, 48 + i) , False, False

deleted accidental repost

@protonLeah,

You are correct... my bad.