Could someone help me with why this leaderboard script will not work?

This is happening because end) on line 28 closes your event listener. that end) at the bottom is not closing off anything and that is why it gives you an error. You should move lines 29 and 35 inside your event listener.

local ds = game:GetService("DataStoreService"):GetDataStore("--Cash01")

game.Players.PlayerAdded:Connect(function(player)
    local key = "cash-"..player.UserId
    local folder = Instance.new("Folder") -- # parent objects last for better performance
    folder.Name = "leaderstats"
    folder.Parent = player

    local currency = Instance.new("IntValue")
    currency.Name = "Coins"
    currency.Value = ds:GetAsync(key) -- # defaults to 0 if there isn't any data 
    currency.Parent = folder

    local fake1 = Instance.new("StringValue")
    fake1.Name = currency.Name
    fake1.Value = tostring(currency.Value)
    fake1.Parent = folder

    currency.Changed:Connect(function()
        local x = tostring(currency.Value)
        if #x >= 10 then -- Billions
            fake1.Value = x:sub(0,(#x-9)).."."..(x:sub(#x-7,(#x-7))).."B+"
        elseif #x >= 7 then -- Millions
            fake1.Value = x:sub(0,(#x-6)).."."..(x:sub(#x-5,(#x-5))).."M+"
        elseif #x >= 4 then -- Thousands
            fake1.Value = x:sub(0,(#x-3)).."."..(x:sub(#x-2,(#x-2))).."K+"
        else -- If it's not high enough...
            fake1.Value = tostring(currency.Value)
        end
    end)
end)

Notice the script does not save each time the currency value changes. You should not be doing that. I also removed the infinite loop as you only need to check each time the currency changes. Not the default interval, which is ~0.03 seconds.