Attempt to index nil with 'FindFirstChild'?

I'm making a zombie game with food and thirst systems. What I'm doing now is making so that the value of one's hunger or thirst is equal to the text of the GUI.

local plr = game.Players.LocalPlayer

while true do
	local systems = plr.Character:FindFirstChild("FoodAndThirstSystems")
	local thirstval = systems:FindFirstChild("ThirstVal")
	script.Parent.Text = "Hunger: " ..(tostring(thirstval.Value))
	if thirstval.Value < 10 then
		script.Parent.BackgroundColor3 = Color3.new(0.666667, 0, 0)
	else 
		script.Parent.BackgroundColor3 = Color3.new(0.333333, 1, 1)
	end
	wait(.1)
end

I have the part where the text of the GUI is equal to the Value of one's thirst value.

But in line 5, it has an error that always says " Attempt to index nil with 'FindFirstChild' " I've tried doing "systems.ThirstVal" but it still shows up with: Attepmt to index nil with 'ThirstVal'.

Please tell me what to do. Thanks!

Welp nvm i just made it work.