How can i fix this problem?

How can i make this if the Character's Torso is close to the door the PointLight turns on and when the Character's Torso is farther than 15 then it turns off. The problem is that it doesn't turn on...

local Door = script.Parent

while true do
for i,v in pairs(game.Players:GetChildren()) do
if (v.Character.Torso.Position - Door.Position).magnitude <= 15 then
Door.PointLight.Enabled = true
if (v.Character.Torso.Position - Door.Position).magnitude <= -15 then
Door.PointLight.Enabled = false
end
end
end
wait()
end

Logic

There's two fundamental issues here. Magnitude will never be less than 0, and your if statements are nested with mutually exclusive conditions.

The solution is to use an else statement, which is the compliment of the if statements preceding it. In other words, if your condition isn't true then it goes to the else block.

local Door = script.Parent

while wait() do
	for i,v in pairs(game.Players:GetChildren()) do
		if (v.Character.Torso.Position - Door.Position).magnitude <= 15 then
			Door.PointLight.Enabled = true
			break -- We don't actually want it turning off because nobody else is near (Do you?)
		else
			Door.PointLight.Enabled = false
		end
	end
end

A quick note to you though - This will break when a new Player tries to join, as the Player exists before the Character on a real server.