adding column dynamically depending on date || Complex problem

Hello - this is how you can do it.  Note, in your description you say that the expected result should be a new column which is populated when the month and day from the date column is 12/31, however in the example of the expected result, for John, the new columns are blank for 12/31/2021 and populated for 8/31/2022.  This solution has John's row for 12/31/2021 populated and blank for 8/31/2022.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45W8srPyFPSUTI00jc21DcyMDIEcszMgIRvakpmaa5SrA5cjYEFVI0RkGNuDiQ8MtMzwCpcEssyU0BKzPSNDWBKLIDYJ78crCC4IDUvvyoRpMQSqsQYZIoBNlPgjgGZYmIEMyYWAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Person = _t, Date = _t, Sales = _t, #"Level of sales" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Person", type text}, {"Date", type date}, {"Sales", Int64.Type}, {"Level of sales", type text}}),
    #"Added Custom" = Table.AddColumn(#"Changed Type", "Level of Sales EOY 21", each if Date.ToText ( [Date], "M/dd/yyyy" ) = "12/31/2021" then [Level of sales] else null, type text),
    Custom1 = Table.AddColumn(#"Added Custom", "Level of Sales EOY 22", each if Date.ToText ( [Date], "M/dd/yyyy" ) = "12/31/2022" then [Level of sales] else null, type text)
in
    Custom1