Why doesn't DataStoreService save mutated data?

I'm a complete beginner in Roblox scripting and not very knowledgeable on DataStoreService and leaderstats. If you look at my profile you'll see some posts from like a year ago, because I quit Roblox scripting and started scripting again recently.

When I mutate data in leaderstats it doesn't get saved, even though no errors and/or warnings appear in the console while I'm playing my game.

Here's the Script that uses DataStore in ServerScriptService. I followed this tutorial: https://www.youtube.com/watch?v=hWhr8ntzq5M

local DataStoreService = game:GetService("DataStoreService")

local DataStore = DataStoreService:GetDataStore("PlayerData")

local Players = game:GetService("Players")

local function onPlayerJoin(player)
    local leaderstats = Instance.new("Folder")
    leaderstats.Name = "leaderstats"
    leaderstats.Parent = player

    local coins = Instance.new("IntValue")
    coins.Name = "Coins"
    coins.Parent = leaderstats

    local multiplier = Instance.new("IntValue")
    multiplier.Name = "Multipliers"
    multiplier.Parent = leaderstats

    local playerUserId = "Player_" .. player.UserId
    local data = DataStore:GetAsync(playerUserId)

    print(data)

    if data then
        coins.Value = data["Coins"]
        multiplier.Value = data["Multipliers"]
    else
        coins.Value = 0
        multiplier.Value = 1
    end
end

local function create_table(player)
    local player_stats = {}
    for _, stat in pairs(player.leaderstats:GetChildren()) do
        player_stats[stat.Name] = stat.Value
    end
    return player_stats
end

local function onPlayerLeave(player)
    local player_stats = create_table(player)

    local success, errormessage = pcall(function()
        local playerUserId = "Player_" .. player.UserId
        DataStore:SetAsync(playerUserId, player_stats)
    end)

    if success then
        print("Data Saved")
    elseif not success then
        print("Could not save data")
        warn(errormessage)
    end
end

Players.PlayerAdded:Connect(onPlayerJoin)
Players.PlayerRemoving:Connect(onPlayerLeave)

and the LocalScript that increases the coins in StarterPlayer/StarterCharacterScripts

local playerModel = script.Parent
local Player = game.Players:GetPlayerFromCharacter(playerModel)
local Mouse = Player:GetMouse()

Player:WaitForChild("leaderstats")
Player.leaderstats:WaitForChild("Coins")

local coins = Player.leaderstats.Coins
local multipliers = Player.leaderstats.Multipliers

local function onLeftMouseButtonDown()
    coins.Value += 1 * multipliers.Value
end

Mouse.Button1Down:Connect(onLeftMouseButtonDown)