indefinido

Hola @rima_ch ,
Vaya a power query y péguelo en el editor avanzado:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjTSN9Q3MjC0VNIBsY1BHCMTpdhYAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Start = _t, End = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Start", type date}, {"End", type date}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Dates", each {Number.From([Start])..Number.From([End])}),
    #"Expanded Dates" = Table.ExpandListColumn(#"Added Custom", "Dates"),
    #"Changed Type1" = Table.TransformColumnTypes(#"Expanded Dates",{{"Dates", type date}}),
    #"Added Custom1" = Table.AddColumn(#"Changed Type1", "Year", each Date.Year([Dates])),
    #"Added Custom2" = Table.AddColumn(#"Added Custom1", "Month", each Date.MonthName([Dates])),
    #"Changed Type2" = Table.TransformColumnTypes(#"Added Custom2",{{"Year", type text}, {"Month", type text}}),
    #"Removed Other Columns" = Table.SelectColumns(#"Changed Type2",{"Year", "Month"}),
    #"Grouped Rows" = Table.Group(#"Removed Other Columns", {"Year", "Month"}, {{"det", each _, type table [Year=nullable text, Month=nullable text]}}),
    #"Removed Columns" = Table.RemoveColumns(#"Grouped Rows",{"det"})
in
    #"Removed Columns"

Hágamelo saber si tiene alguna pregunta.

Si esto resuelve sus problemas, márquelo como la solución, para que otros puedan encontrarlo fácilmente. Felicitaciones 👍también son agradables.
Nathaniel

Otra posible construcción:

let
    Start = #date(2019,12,1),
    End = #date(2024,12,31),
    Years = {Date.Year(Start)..Date.Year(End)},
    #"Converted to Table" = Table.FromList(Years, Splitter.SplitByNothing(), {"Year"}),
    #"Added Months" = Table.AddColumn(#"Converted to Table", "Month", each {1..12}),
    #"Expanded Month" = Table.ExpandListColumn(#"Added Months", "Month"),
    #"Changed Type" = Table.TransformColumnTypes(#"Expanded Month",{{"Year", Int64.Type}, {"Month", Int64.Type}}),
    #"Added Date" = Table.AddColumn(#"Changed Type", "Date", each #date([Year], [Month], 1), type date),
    #"Filtered Rows" = Table.SelectRows(#"Added Date", each [Date] >= Start and [Date] <= End),
    #"Added Custom" = Table.AddColumn(#"Filtered Rows", "MonthName", each Date.MonthName([Date]), type text)
in
    #"Added Custom"

Puede mantener o eliminar las columnas adicionales como desee.