Why does it randomly say "attempt to index a nil value?"

I am trying to use a script where a billboard gui pops up on the closest part to the player that the player spawned using a button. I tried using this script:

local plrName = script.Parent.Name
local rK = game.Players:GetPlayerFromCharacter(script.Parent).PlayerGui:WaitForChild("RKeyGen")
n = nil

while true do
	for i, v in pairs(game.Workspace.Bricks:GetChildren())do
		if v.Name == plrName then
			local pPos = v.Position
			local lPos = script.Parent:FindFirstChild("HumanoidRootPart").Position
			local sAway = (pPos - lPos).magnitude
			if sAway ~= nil then
				v:FindFirstChild("NumVal").Value = tonumber(sAway)
				print(sAway)
				if n ~= nil then
					if v:FindFirstChild("NumVal").Value > n then
						rK.Enabled = true
						rK.Adornee = v
					else
						n = v:FindFirstChild("NumVal").Value
					end
				else
					n = v:FindFirstChild("NumVal").Value
				end
			end
		end
	end
	wait(0.15)
end

It works sometimes, but then an error occurs randomly and says: "Workspace.snaks308070.Script:12: attempt to index a nil value"

Any help?

If you get any errors, and it says : with a number, it's that script line. A nil value error means that it can't find what you are localizing. Use waitforchild instead of findfirstchild in that case so that instead of trying to find it and finding nothing, it will wait until it comes, if you want a different outcome comment