if text.length

[Miss_A]

= Table.AddColumn(#"Renamed Columns", "Custom", each if Text.Length[MonthYear]=6 then "0"&[MonthYear] else [MonthYear])

When I write the formula for 1-2023 to 01-2023

Expression.Error: We cannot apply field access to the type Function.
Details:
Value=[Function]
Key=MonthYear

[Syndicate_Admin]

You forgot some parentheses

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMtQ1MjAyVorVATJh7FgA", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [MonthYear = _t]),
    #"Added Custom" = Table.AddColumn(Source, "Custom", each if Text.Length([MonthYear])=6 then "0"&[MonthYear] else [MonthYear])
in
    #"Added Custom"

[Fowmy · Accepted solution]

@Miss_A 

This method should work for you.

Date.ToText(Date.FromText( "1-" &  [Column1],"en-gb"), "MM-yyyy")