GUI functions when there is only one player in game but breaks if another joins?

EDIT 2 I found it, when using DataStore2, you must use the dot operator when using Combine like this:

DataStore2.Combine("MasterKey", "Store1", ... ,"StoreN")

But I had been using the colon operator like this:

DataStore2:Combine("MasterKey", "Store1", ... ,"StoreN")

EDIT: Upon further examination, I believe the issue is with the DataStore2 module I'm using, not these scripts themselves. When I test the game on a local server, upon the second player joining I get the following output:

 15:26:49.988 - ServerScriptService.DataStore2:479: DataStore2() API call expected {string dataStoreName, Instance player}, got {table, Instance}
15:26:50.001 - Stack Begin
15:26:50.004 - Script 'ServerScriptService.DataStore2', Line 479 - function __call
15:26:50.007 - Script 'ServerScriptService.DataStore2', Line 491 - function __call
15:26:50.008 - Script 'ServerScriptService.MainDataStore', Line 8
15:26:50.009 - Stack End

Not quite sure what this means but I'm going to keep looking.

OG POST

I'm having trouble figuring out what is wrong with these scripts, because I have other GUI's which function perfectly regardless of how many players are in game. This is the only one which is giving me trouble and I can't figure it out.

So there is a button when pressed should award the player a random amount of cash from 1 to 6. It works with one player in-game but as soon as another joins, it completely breaks.

This is the local script which is inside a button:

local rollEvent = game:GetService("ReplicatedStorage"):WaitForChild("rollEvent") --remote event to fire to server script
local CooldownBar = script.Parent.Parent.CooldownBar --this is to show the remaining time before button can be clicked again
local CooldownText = CooldownBar.TimeRemaining --this too
local bar = CooldownBar.ActualBar --this too

local debounce = false --debounce for the cooldown

local player = game.Players.LocalPlayer --gets player to send to server script

script.Parent.MouseButton1Click:Connect(function()

    if debounce then --debounce true then dont do anything
        return
    end

    debounce = true --else set it to true and continue

    rollEvent:FireServer(player) --fire to server script

    local rollCooldown = game.Players.LocalPlayer.vars.rollCooldown --gets the cooldown value from player data

    bar.Size = UDim2.new(0, 0, .8, 0) --controls cooldown bar size to show when you can click button again
    bar:TweenSize(UDim2.new(.975, 0, .8, 0), "Out", "Linear", rollCooldown.Value, true)

    local secondsToRun = rollCooldown.Value
    local endTime = tick() + secondsToRun -- float representing when the timer was started
    while tick() < endTime do -- run until 'secondsToRun' seconds have passed
        game:GetService("RunService").Heartbeat:Wait() -- this waits until the heartbeat event is triggered, which happens 60 times a second
        local timeSinceStart = endTime - tick()
        local numberText = string.format("%0.1f", tostring(timeSinceStart)) -- 0.1 represents one decimal
        CooldownText.Text = numberText
    end

    CooldownText.Text = "Ready!"

    debounce = false --button can now be clicked again

end)

And this is the server sided script in server script service:

local rollEvent = game:GetService("ReplicatedStorage"):WaitForChild("rollEvent")
local debounce = false

rollEvent.OnServerEvent:Connect(function(player) --when event is fired, do the function

    if debounce then --just to prevent exploits
        return
    end

    debounce = true

    local vars = player:WaitForChild("vars")
    local cash = vars:WaitForChild("Cash")
    local rollCooldown = vars:WaitForChild("rollCooldown")
    local lastRoll = player:WaitForChild("lastRoll")

    local chance = math.random(1,6)

    local cashDataStore = dataStore2("Cash", player) --get cash datastore
    cashDataStore:Increment(chance) --update the value

    lastRoll.Value = chance --sent back to client to show how much the last click gave them
    rollEvent:FireClient(player, lastRoll)

    wait(rollCooldown.Value - 0.05)

    debounce = false

end)

I honestly cannot figure it out but knowing me, it's probably something really dumb or minor that I just can't find. If you need any more information or scripts I can provide it.