How to use expression (x,y,z)

Thanks a lot.

I was trying to write a custom function based on this guide.

My target is to use this custom function for different scope, this is the reason why I want to keep it as general as possible.

However I already met the first problem. 

What I wrote is:

(
Input_Table as table,
InputColumn1 as text,
InputColumn2 as text
) =>

let
AddIndex = Table.AddIndexColumn (Input_Table, "Index", 1,1),
UnPivotedTable = Table.UnpivotOtherColumns (AddIndex, {"InputColumn1", "InputColumn2"}, "Attribute", "Value")

in UnPivotedTable

I  invoked it in this way:

let
Source = Query1(Summary_Volumes, "Body_Material", "Struffing_Box_Material")
in
Source

This gives an error message I really don't understand:

Could you help me in understanding why this is not working?

Thanks a lot in advance