How to add additional row with dates in Power Query

I think I understand what you want, and I believe the following will provide it, based only on the five columns you show.

I believe I have commented the code  enough so you can understand it, but ask questions if you have them.

Main Code

let

//edit to reflect your actual data source
    Source = Excel.CurrentWorkbook(){[Name="Table3"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{
        {"Article number", Int64.Type}, {"End_date", type date}, {"Old buying price", type number}, 
        {"Start_date", type date}, {"New buying price", type number}}),

//Group by article number
// then run custom function on each subtable
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Article number"}, {
        {"Count", each fnSemiMonthly(_)}}),

//remove unneeded column and expand the subtables, renaming the columns
    #"Removed Columns" = Table.RemoveColumns(#"Grouped Rows",{"Article number"}),
    #"Expanded Count" = Table.ExpandTableColumn(#"Removed Columns", "Count", 
        {"End_date", "Article number", "Old buying price"}, 
        {"Date", "Article number", "Price"}),

//set the data types
    #"Changed Type1" = Table.TransformColumnTypes(#"Expanded Count",{
        {"Date", type date}, {"Article number", Int64.Type}, {"Price", type number}}),

//set desired column order
    #"Reordered Columns" = Table.ReorderColumns(#"Changed Type1",{"Article number", "Date", "Price"})
in
    #"Reordered Columns"

Custom Function

//Rename fnSemiMonthly

(tbl as table)=>

let
    Source = tbl,

//create a list (then make it a table) of all dates from 2022.12.01 to the later of Today vs the last date in the table, for the1st and 15th of each month
    #"Semi Monthly Dates" = Table.FromList(List.Generate(
        ()=>[d=#date(2020,12,1)],
        each [d] <= List.Max({List.Max(Source[Start_date]),Date.From(DateTime.LocalNow())}),
        each [d=if Date.Day([d]) = 1 then Date.AddDays([d],14) else Date.StartOfMonth(Date.AddMonths([d],1))],
        each [d]), Splitter.SplitByNothing(), type table[End_date=date]),

//Combine with the original table
    #"Combine with Source" = Table.Combine({#"Semi Monthly Dates",Source}),

//Sort by date
//Then fillup to replace the nulls with the first filled in entry
    #"Sorted Rows" = Table.Sort(#"Combine with Source",{{"End_date", Order.Ascending}}),
    #"Filled" = Table.FillUp(#"Sorted Rows",{"Article number", "Old buying price"}),

//Remove duplicates, then fill down new buying price to
//  replace prices in old price column if needed
    #"Removed Duplicates" = Table.Distinct(#"Filled", {"End_date"}),
    #"Filled Down" = Table.FillDown(#"Removed Duplicates",{"New buying price"}),
    #"Replaced Value" = Table.ReplaceValue(#"Filled Down",null,each [New buying price],Replacer.ReplaceValue,{"Old buying price"}),
    #"Removed Columns" = Table.RemoveColumns(#"Replaced Value",{"Start_date", "New buying price"}),

//Fill down to get last article numbers filled in
    #"Filled Down1" = Table.FillDown(#"Removed Columns",{"Article number"}),

//Remove dates that are not the 1st or 15th
    #"Remove Rows" = Table.SelectRows(#"Filled Down1", each Date.Day([End_date]) = 1 or Date.Day([End_date]) = 15)
in
    #"Remove Rows"

Results

Article numberDatePrice