Why is the DataStore not saving correctly?

Wiki's description on PlayerRemoving:

The PlayerRemoving event fires right before a Player is leaves the game. This event fires before Instance/ChildRemoved|ChildRemoved does on Players, and behaves somewhat similarly to Instance/DescendantRemoving. Since event fires before the actual removal of a Player, this event is useful to store player data using a GlobalDataStore.

So the data was never saving because it was too late, the player already left. Simply switching the events should fix the problem. Always use PlayerAdded and PlayerRemoving on the Players service. Another thing is that ChildAdded and ChildRemoved also fires for non-Player instances.

Now, for the code itself, it is way too long. You should use loops.

local Datastore = game:GetService("DataStoreService"):GetDataStore("Seraph1V5!")
local ServerStorage = game:GetService("ServerStorage")
local names = {"Race", "Hair", "Shirt", "Pants", "Colour"}
local Noob = {
    Seraph = false,
    Tries = 3,
    Race = "Human",
    Hair = "Beautiful Hair",
    Shirt = "Yu",
    Pants = "Yu",
    Colour = "Pastel Brown",
    LuckyNumber = 0,
    CharacterCreated = false,
}

local function PlayerAdded(player) -- # localise your variables
    local stringValues = {} -- # store our string values 
    local key = "user "..player.UserId
    local Persistence = Instance.new("Folder") -- 2nd argument discouraged
    Persistence.Name = "Persistence"..player.Name
    Persistence.Parent = ServerStorage -- # Parent last

    local Data = Instance.new("Folder")
    Data.Name = "Data"
    Data.Parent = Persistence

    local Seraph = Instance.new("BoolValue")
    Seraph.Name = "Seraph"
    Seraph.Parent = Data

    local CharacterCreated = Instance.new("BoolValue")
    CharacterCreated.Name = "CharacterCreated"
    CharacterCreated.Parent = Data

    local Tries = Instance.new("IntValue")
    Tries.Name = "Tries"
    Tries.Parent = Data

    local LuckyNumber = Instance.new("IntValue") -- # it's a number, should be IntValue
    LuckyNumber.Name = "LuckyNumber"
    LuckyNumber.Parent = Data

    for _, v in ipairs(names) do
        -- # create stringvalue each name 
        local stringValue = Instance.new("StringValue")
        stringValue.Name = v
        stringValue.Parent = Data
        table.insert(stringValues, stringValue) -- # insert string value into stringValues array
    end

    local StoredData = Datastore:GetAsync(key) or Noob

    Seraph.Value = StoredData.Seraph
    Tries.Value = StoredData.Tries
    LuckyNumber.Value = StoredData.LuckyNumber
    CharacterCreated.Value = StoredData.CharacterCreated

    for k, v in pairs(StoredData) do
        for _, stringValue in ipairs(stringValues) do
            if type(v) == "string" then -- # assign string value Value property to the strings
                stringValue.Value = v
            end
        end
    end
end 
----------------------------------------------------------- 

local function PlayerLeaving(player)
    local key = "user "..player.UserId
    local dataToStore = {}

    for _, child in ipairs(ServerStorage["Persistence" .. player.Name]:GetChildren()) do
        dataToStore[child.Name] = child.Value -- # key is the name of child, value is it's Value property
    end

    local success, message = pcall(function()
        Datastore:SetAsync(key, dataToStore)
    end)
    if success then
        print("Saved '"..player.Name.."' Data.")
    end
end


game.Players.PlayerAdded:Connect(PlayerAdded)
game.Players.PlayerRemoving:Connect(PlayerLeaving)
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