Leaderstats value doesn't save upon adding a value?

Hello,

I'm currently working on an RPG MMO game, and I'm trying to make a quest system with an exchange. For example: Earn 20 gold for 300 xp. So I have a LocalScriptin the TextButton in a ScreenGui and the script is:

local player = game.Players.LocalPlayer

script.Parent.MouseButton1Click:Connect(function()
    if player.leaderstats.MobKills.Value >= 50 then
        player.leaderstats.Coins.Value = player.leaderstats.Coins.Value + 10000
        player.leaderstats.MobKills.Value = player.leaderstats.MobKills.Value - 50
        script.Parent.TextSize = 22
        script.Parent.Text = "Quest Commpleted"
        script.Disabled = true
    end
end)

So if the player has 50 MobKills then they can exchange for 10000 Coins, this works and it updates, however it doesn't save, so when the player rejoins the game it will go back to what it was before if they hadn't clicked on the quest.

I'm not sure why it doesn't save, and I would really appreciate some help. Thanks!