why isn't this script working even though there was no error?

In Lua the -= and += operators are not defined. You need to write it as

hum.Health = hum.Health - 1

Also the repeat until 9999... is quite messy. Instead, use while wait(1) do instead since you are placing the wait in the repeat anyways.

while wait(1) do
	for i, player in ipairs(game.Players:GetPlayers()) do
		if player.Character then
			local hum = player.Character:FindFirstChild('Humanoid')
			hum.Health =  hum.Health - 1
		end
	end
end

For the next script I'm assuming it would be a food item or something in the workspace. You would need a clickdetector on the 'food' item.

I would place a script into the part or food that will be clicked on.

local ClickDetector = Instance.new("ClickDetector")
ClickDetector.Parent = script.Parent
ClickDetector.MaxActivationDistance = 5 --player has to be close to eat

local click = script.Parent.ClickDetector

click.MouseClick:Connect(function(player)
	if player.Character then
		local hum = player.Character:FindFirstChild('Humanoid')
		hum.Health = hum.Health + 10
	end
	script.Parent:Destroy()
end)