How can i fix this problem?

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
elseif (v.Character.Torso.Position - Door.Position).magnitude >= 15 then
Door.PointLight.Enabled = false
end
end
wait()
end

This is a working version of your script I just changed the magnitude to greater than 15 instead of less than -15 and I changed your if to an elseif statement.

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.