question

The value for the key “CurrentSeason” has to be a string in order for PlayFab to handle. Therefore, you should convert that JavaScript object into a string, for instance, with JSON.stringify(), before parsing it to the key “CurrentSeason”. I made some changes to your code, which works as expected:

handlers.newplayer = function (args, context) {
    var TitleData = server.GetTitleData({Keys: null});    var CurrentSeason = "Season_" + TitleData.Data["CurrentSeason"];    //New Player Data
    var dataRequest = {
        PlayFabId: currentPlayerId, 
        Data: {
            "Level": 2,
            [CurrentSeason]: JSON.stringify({
                "Rank":0,
                "RemainingPlacementMatch":TitleData.Data["NeededPlacementMatch"]
            })
        }
    };
    server.UpdateUserData(dataRequest);      return { "Data": dataRequest };
}