Count down for round timer is working but troubleshooting in output??

EDIT: Just realised I made an error and did "WaitForChild" instead of FindFirstChild

I had the same problem earlier today. I'm assuming you're using R15, in R6, this would show no errors. My way to fix it was to wait for it to load so:

local txt = {"Time left until next round: "}

function startPr()
    for j = 300, 0, -1 do
        for i,v in pairs(game.Players:GetChildren()) do
            if v:FindFirstChild("PlayerGui") then
                if v.PlayerGui:FindFirstChild("ScreenGui") then
                    v.PlayerGui.ScreenGui.Frame.TextLabel.Text = txt[1]..j
                end
            end
        end
        wait(1)
    end
end

startPr()