Why it won't save cash to DataStore?

In line 20, you only specified the key, not the value. You need to provide the value for it to set as well!

local DataStore = game:GetService("DataStoreService"):GetDataStore("CashStore")

game.Players.PlayerAdded:connect(function(player)
    if script.Parent.SettingsTab.CashSave.Value == true then
        pcall(function()
            local store = DataStore:GetAsync("u_"..player.UserId) -- Also, stop using deprecated stuff! It is a bad habit!
            if store then
                print("Found cash data")
                PlayerStats.Value = store
            else
                print ("No data found")
            end
        end)
    end
end)

game.Players.PlayerRemoving:connect(function(player)
    if script.Parent.SettingsTab.CashSave.Value == true then
        pcall(function()
            DataStore:SetAsync("u_"..player.UserId, PlayerStats.Value) -- There, this should work!
            print("Saved")
        end)
    end
end)

Any questions? Please leave a comment below. Thanks!