Previous Latest Date

Hi. This is not the best place to do it if you have a database as source because it has to query itself. Let's see. Add a custom column and insert the statement after "LastD", the final code should look like this:

= Table.AddColumn(#"Last Step", "LastD", 
    (Earlier) => 
        Table.Max(
             Table.SelectRows(
                Table.SelectColumns(#"Changed Type", {"Date", "CIF"})
            , each Earlier[Date] > [Date] and [CIF] = Earlier[CIF] 
        ) 
        , "Date", Earlier 
    )[Date] 
)

 Hope this helps,

I am not clear if and how your problem was solved.
but in any case try if this is right for you

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("bczBCcAwDEPRXXwOWJLTaUz2X6MtPgSHXD9PyjSSNkzhoAuCrZEm6Y+zxZKU4zklo8WS3xbzlFCLJQOXT3HH9QI=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [CIF = _t, DATE = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"CIF", Int64.Type}, {"DATE", type date}}),

    maxSmaller = (lst, el)=> 
    let 
    ls=List.Sort(lst)
    in ls{List.Max({List.PositionOf(ls,el)-1,0})},


     ms = (tab)=> Table.AddColumn(tab, "maxSmaller", each maxSmaller(tab[DATE], _[DATE])),

    
    #"Grouped Rows" = Table.Group(#"Changed Type", {"CIF"}, {{"ms", each ms(_)}}),
    #"Expanded ms" = Table.ExpandTableColumn(#"Grouped Rows", "ms", {"DATE", "maxSmaller"}, {"DATE", "maxSmaller"})
in
    #"Expanded ms"