accumulated of zero

See the working here - Open a blank query - Home - Advanced Editor - Remove everything from there and paste the below code to test

let
    Fonte = Excel.Workbook(File.Contents("C:\Users\alex\Desktop\\stock_acumulated.xlsx"), null, true),
    stock_Table = Fonte{[Item="stock",Kind="Table"]}[Data],
    #"start" = Table.TransformColumnTypes(stock_Table,{{"code", Int64.Type}, {"date", type date}, {"trans_id", Int64.Type}, {"Index", Int64.Type}, {"amount", Int64.Type}, {"day amount", Int64.Type}}),
    ListOfIndex = List.Buffer(#"start"[Index]),
    ListOfamount = List.Buffer(#"start"[amount]),
    //Function Start
    fxGetAccum=(ListOfIndex, ListOfamount)=>
        let
            FunctionResult = List.Generate(()=>[x=ListOfamount{0},i=0], each [i]<List.Count(ListOfIndex), each [i=[i]+1, x=(if ListOfIndex{i}=0 then 0 else [x]) + ListOfamount{i}], each [x])
        in
            FunctionResult,
    //Function End
    Result = Table.FromColumns(Table.ToColumns(#"start")&{fxGetAccum(ListOfIndex, ListOfamount)},Table.ColumnNames(#"start")&{"Accumulated"})
in
    Result