Left outer join by combing two tables

Your expected output cannot be achieved with just a Left Outer Join. 

= Table.NestedJoin(#"Table 1", {"Month Year ", "Cat "}, #"Table 2", {"Month Year ", "Cat "}, "Table 2", JoinKind.LeftOuter)

Looks more like you wanted a cross join between [Month Year] and [Cat]  of Table 1 and then a left outer join with table 2.

= Table.NestedJoin(Table.ExpandTableColumn(Table.AddColumn(Table.Distinct(Table.SelectColumns(#"Table 1",{"Month Year "})), "Custom", each Table.Distinct(Table.SelectColumns(#"Table 1",{"Cat "}))), "Custom", {"Cat "}, {"Cat "}), {"Month Year ", "Cat "}, #"Table 2", {"Month Year ", "Cat "}, "Table 2", JoinKind.LeftOuter)

let
    Source = Table.NestedJoin(Table.ExpandTableColumn(Table.AddColumn(Table.Distinct(Table.SelectColumns(#"Table 1",{"Month Year "})), "Custom", each Table.Distinct(Table.SelectColumns(#"Table 1",{"Cat "}))), "Custom", {"Cat "}, {"Cat "}), {"Month Year ", "Cat "}, #"Table 2", {"Month Year ", "Cat "}, "Table 2", JoinKind.LeftOuter),
    #"Expanded Table 2" = Table.ExpandTableColumn(Source, "Table 2", {"Code", "Date"}, {"Code", "Date"}),
    #"Sorted Rows" = Table.Sort(#"Expanded Table 2",{{"Cat ", Order.Ascending}, {"Month Year ", Order.Ascending}})
in
    #"Sorted Rows"