What about assigning unique values with multiple criteria in power BI as like , for instance.

See the working here - Open a blank query - Home - Advanced Editor - Remove everything from there and paste the below code to test

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjEwMDA1tzQ3UNJRsjAws7AwMTRXitUhTcICRcIQIWGJQ8LIgDijjBA6DEmWMMIlYYxLwkQpNhYA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Purchase Order" = _t, #"Delivery no." = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Purchase Order", Int64.Type}, {"Delivery no.", Int64.Type}}),
    #"Added Index" = Table.AddIndexColumn(#"Changed Type", "Index", 0, 1, Int64.Type),
    #"Grouped Rows" = Table.Group(#"Added Index", {"Purchase Order"}, {{"All", each _}}),
    #"Added Custom" = Table.AddColumn(#"Grouped Rows", "Custom", each let
DummyTbl = [All],
output = Table.AddColumn(DummyTbl,"Result",each List.PositionOf(List.Distinct(DummyTbl[#"Delivery no."]),[#"Delivery no."])+1)
in
output),
    #"Removed Columns" = Table.RemoveColumns(#"Added Custom",{"All"}),
    #"Expanded Custom" = Table.ExpandTableColumn(#"Removed Columns", "Custom", {"Delivery no.", "Index", "Result"}, {"Delivery no.", "Index", "Result"}),
    #"Sorted Rows" = Table.Sort(#"Expanded Custom",{{"Index", Order.Ascending}}),
    #"Removed Columns1" = Table.RemoveColumns(#"Sorted Rows",{"Index"}),
    #"Changed Type1" = Table.TransformColumnTypes(#"Removed Columns1",{{"Purchase Order", Int64.Type}, {"Delivery no.", Int64.Type}, {"Result", Int64.Type}})
in
    #"Changed Type1"

Thank you vijay, your solution is successfully worked.