Parent-Child Hierarchy - Highest Parent (using Power Query not DAX)

you just need to change the data types then like this: 

let
    Source = Excel.CurrentWorkbook(){[Name="Hierarchy"]}[Content],

    ChangedType = Table.TransformColumnTypes(Source,{{"Team ID", type text}, {"Parent ID", type text}}),

    ListTeamID = List.Buffer( ChangedType[Team ID] ),
    ListParentID = List.Buffer( ChangedType[Parent ID] ),


    fnGetHighestParent =  (n as text) as text =>
        let
            PosOfParent = List.PositionOf( ListTeamID, n ),
            ParID = ListParentID{PosOfParent}
         in
            if ParID = null then ListTeamID{PosOfParent} else @fnGetHighestParent(ListParentID{PosOfParent}),


    FinalTable = Table.AddColumn( ChangedType, 
                                  "HighestParent", 
                                  each  fnGetHighestParent( [Team ID] ), 
                                   type text)
    
in
   FinalTable