Adding values from table to list

Try this code RvanMelis - [Start Date] cannot be directly accessed within the List.Transform function that way, so you need to capture it as a variable before List.Transform, then use the variable.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMtQ31DcyMDJU0lEyVIrViVYyQQgYK8XGAgA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [StartDate = _t, #"Months Left" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"StartDate", type date}, {"Months Left", Int64.Type}}),
    #"Added Custom" = 
        Table.AddColumn(
            #"Changed Type", 
            "Test Column", 
            each
                let
                    varStartDate = [StartDate]
                in
                List.Transform(
                    {0..[Months Left]}, 
                    each Date.AddMonths(varStartDate, _)
                )
            )
in
    #"Added Custom"

How to use M code provided in a blank query:
1) In Power Query, select New Source, then Blank Query
2) On the Home ribbon, select "Advanced Editor" button
3) Remove everything you see, then paste the M code I've given you in that box.
4) Press Done
5) See this article if you need help using this M code in your model.