Creating Conditional Date Column based on Individual Client ID Variable

You can group by ClientID and take the min over the date column to find the Initial dates. Then merge that back with the original query and add a custom column that returns different results if the date equals the initial date.

Here's a full sample query you can paste into the Advanced Editor of a new blank query:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("VczLCQAgEAPRXnIWYlZFrUW2/zb8HcTrg5kxIASIirSoDg9XGtMC0wFbkKkfKmWvSfsili0N7hM=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [ClientID = _t, LastPRAPAREDate = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"ClientID", Int64.Type}, {"LastPRAPAREDate", type date}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"ClientID"}, {{"InitialDate", each List.Min([LastPRAPAREDate]), type nullable date}}),
    #"Merged Queries" = Table.NestedJoin(#"Changed Type", {"ClientID"}, #"Grouped Rows", {"ClientID"}, "Grouped Rows", JoinKind.LeftOuter),
    #"Expanded Grouped Rows" = Table.ExpandTableColumn(#"Merged Queries", "Grouped Rows", {"InitialDate"}, {"InitialDate"}),
    #"Added Custom" = Table.AddColumn(#"Expanded Grouped Rows", "AssementType", each if [LastPRAPAREDate] = [InitialDate] then "Initial" else "Reassessment", type text)
in
    #"Added Custom"