Column with accumulated values

It seems you are defining "last two months" as relating solely to the row position. Otherwise, why is the last row "1270" and not "1029"?  Or are the dates in "Start of Month" in the wrong order?

To obtain the output you show from the input you provide, try the code below:

let
    Source = Excel.CurrentWorkbook(){[Name="Table32"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{
            {"Start of Month", type date}, {"SKU", type text}, {"Quantity", Int64.Type}, 
            {"Sales Order Invoiced Qty", Int64.Type}, {"Sales Order Qty", Int64.Type}, 
            {"ERROR", Int64.Type}}),
    
    #"Grouped Rows" = Table.Group(#"Changed Type", {"SKU"}, {
        {"Error 2m", (t)=> let 
            #"2MRT" = List.Generate(
                ()=>[rt=t[ERROR]{0}, idx=0],
                each [idx] < Table.RowCount(t),
                each [rt=t[ERROR]{[idx]} + t[ERROR]{[idx]+1}, idx=[idx]+1],
                each [rt]),
            #"add 2MRT" = Table.FromColumns(
                    Table.ToColumns(t) & {#"2MRT"},
                    Table.ColumnNames(#"Changed Type") & {"ERROR 2M"}
            )
            in #"add 2MRT",
             type table [Start of Month=nullable date, SKU=nullable text, 
                 Quantity=nullable number, Sales Order Invoiced Qty=nullable number, 
                 Sales Order Qty=nullable number, ERROR=nullable number, ERROR 2M=nullable number]}}),
    
    #"Removed Columns" = Table.RemoveColumns(#"Grouped Rows",{"SKU"}),
    #"Expanded Error 2m" = Table.ExpandTableColumn(#"Removed Columns", "Error 2m", 
        {"Start of Month", "SKU", "Quantity", "Sales Order Invoiced Qty", "Sales Order Qty", "ERROR", "ERROR 2M"})
in
    #"Expanded Error 2m"