Save/load data stopped working?

I noticed that you didn't make the stats folder in the player. In PlayerAdded, always make the stats folder, then you can identify it. Also, when getting ad saving the data, I recommend repeating the pcall until it returns success and putting all data in a table when saving it.

And when the server is getting shut down, it kicks the player before it fires the PlayerRemoving event. To avoid that, we'll use the game:BindToClose() function.

local Players = game:GetService("Players")
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local RunService = game:GetService("RunService")
local DataStoreService = game:GetService("DataStoreService")

local remote =ReplicatedStorage:WaitForChild("QuestRemote")
local my_data = DataStoreService:GetDataStore("my_data")

function GetData(player)
    -- getting the data
    local key = player.UserId .. "-player_dataTable"
    local success, dataTable

    repeat
        success, dataTable = pcall(my_data.GetAsync, my_data, key)
    until (success and dataTable) or not Players:FIndFirstChild(player.Name)

    if success then
        -- for stats
        player.stats.Gold.Value = dataTable.stats.Gold or 0
        player.stats.Level.Value = dataTable.stats.Level or 0
        player.stats.Exp.Value = dataTable.stats.Exp or 0

        -for quest stuff
        player.QuestInformation.QuestTitle.Value = dataTable.questInfo.QuestTitle or ""
        player.QuestInformation.QuestTask.Value = dataTable.questInfo.QuestTask or ""
        player.QuestInformation.QuestObjective.Value = dataTable.questInfo.QuestObjective or ""
        player.QuestInformation.QuestProgress.Value = dataTable.questInfo.QuestProgress or 0
        player.QuestInformation.QuestGold.Value = dataTable.questInfo.QuestGold or 0
        player.QuestInformation.QuestExp.Value = dataTable.questInfo.QuestExp or 0

        -- others
        player.Boats_bought.schooner_ship.Value = dataTable.boatsBought.schooner_ship or false

        print("Data has been loaded successfully!")
    end
end

function SaveData(player)
    local stats = player:WaitForChild("stats")
    local questInfo = player:WaitForChild("QuestInformation")
    local boatsBought = player:WaitForChild("Boats_bought")

    -- saving the data
    local key = player.UserId .. "-player_dataTable"
    local dataTable = {
        ["stats"] = {
            Gold = stats:WaitForChild("Gold").Value,
            Level = stats:WaitForChild("Level").Value,
            Exp = stats:WaitForChild("Exp").Value
        },

        ["questInfo"] = {
            QuestTitle = questInfo:WaitForChild("QuestTitle").Value,
            QuestTask = questInfo:WaitForChild("QuestTask").Value,
            QuestObjective = questInfo:WaitForChild("QuestObjective").Value,
            QuestProgress = questInfo:WaitForChild("QuestProgress").Value,
            QuestGold = questInfo:WaitForChild("QuestGold").Value,
            QuestExp = questInfo:WaitForChild("QuestExp").Value
        },

        ["boatsBought"] = {
            schooner_ship = boatsBought:WaitForChild("schooner_ship").Value
        }
    }

    local success, result

    repeat
        success, result = pcall(my_data.UpdateAsync, my_data, key, function(oldData)
            return dataTable
        end)
    until success

    if success then
        print("Data has been saved successfully!")
    end
end

for _, player in ipairs(Players:GetPlayers()) do
    coroutine.wrap(GetData)(player)
end

Players.PlayerAdded:Connect(GetData)
Players.PlayerRemoving:Connect(SaveData)

game:BindToClose(function()
    if RunService:IsStudio() then
        task.wait(2)
    else
        local finished = Instance.new("BindableEvent")
        local allPlayers = Players:GetPlayers()
        local leftPlayers = #allPlayers

        for _, player in ipairs(allPlayers) do
            coroutine.wrap(function()
                SaveData(player)
                leftPlayers -= 1
                if leftPlayers == 0 then
                    finished:Fire()
                end
            end)()
        end

        finished.Event:Wait()
    end
end)