Try:
M-Code
let
Source = Excel.CurrentWorkbook(){[Name="Table20"]}[Content],
#"Changed Type" = Table.TransformColumnTypes(Source,
{{"Country", type text}, {"Year", Int64.Type}, {"Month No", Int64.Type}}),
//Country List
countries = List.Distinct(Table.Column(#"Changed Type","Country")),
//Calculate full date
#"Added Custom" = Table.AddColumn(#"Changed Type", "fullDate", each #date([Year],[Month No],1),type date),
//determine latest month flag by country
#"Grouped Rows" = Table.Group(#"Added Custom", {"Country"}, {{"grouped",
each _, type table [Country=nullable text, Year=nullable number, Month No=nullable number, fullDate=date]},
{"latest fullDate", each List.Max([fullDate]), type date}}),
#"Expanded grouped" = Table.ExpandTableColumn(#"Grouped Rows", "grouped", {"Year", "Month No", "fullDate"}, {"Year", "Month No", "fullDate"}),
#"Added Custom1" = Table.AddColumn(#"Expanded grouped", "Latest Month Flag", each if [latest fullDate] = [fullDate] then 1 else 0),
#"Removed Columns" = Table.RemoveColumns(#"Added Custom1",{"latest fullDate"}),
//Determine latest month flag for ALL countries
#"Grouped Rows1" = Table.Group(#"Removed Columns", {"fullDate"}, {{"Grouped", each _, type table [Country=nullable text, Year=nullable number, Month No=nullable number, fullDate=nullable date, Latest Month Flag=number]}}),
#"Added Custom2" = Table.AddColumn(#"Grouped Rows1", "Latest month ALL countries", each List.Count(
List.RemoveMatchingItems(countries, Table.Column([Grouped],"Country"))) = 0),
#"Grouped Rows2" = Table.Group(#"Added Custom2", {"Latest month ALL countries"}, {{"Grouped", each _, type table [fullDate=nullable date, Grouped=table, Latest month ALL countries=logical]}, {"maxAll", each List.Max([fullDate]), type nullable date}}),
#"Expanded Grouped" = Table.ExpandTableColumn(#"Grouped Rows2", "Grouped", {"fullDate", "Grouped"}, {"fullDate", "Grouped.1"}),
#"Added Custom3" = Table.AddColumn(#"Expanded Grouped", "Latest month ALL countries Flag", each if [maxAll] = [fullDate] and
[Latest month ALL countries] = true
then 1 else 0),
#"Expanded Grouped.1" = Table.ExpandTableColumn(#"Added Custom3", "Grouped.1", {"Country", "Year", "Month No", "fullDate", "Latest Month Flag"}, {"Country", "Year", "Month No", "fullDate.1", "Latest Month Flag"}),
#"Removed Columns1" = Table.RemoveColumns(#"Expanded Grouped.1",{"Latest month ALL countries", "fullDate", "fullDate.1", "maxAll"}),
#"Sorted Rows" = Table.Sort(#"Removed Columns1",{{"Country", Order.Descending}, {"Year", Order.Ascending}, {"Month No", Order.Ascending}})
in
#"Sorted Rows"
Original Data

Results

*Note: If there are no common dates in one of the countries, there will be no flag in the ALL countries column
Latest month available all the country Flag
? Else what is the logic for Aus when11/2020
is not available in other countries – Ron Rosenfeld