Can someone please tell me how this health GUI doesn't regenerate?

The health bar moves just fine when damaged but I want it to regenerate health every 3 seconds or so and I have tried so hard to make it like that but it still refuses to work.

wait(2)
game.Players.LocalPlayer.Character.Humanoid.Changed:connect(function()
local redzone = game.Players.LocalPlayer.Character.Humanoid.MaxHealth / 4
local yellowzone = game.Players.LocalPlayer.Character.Humanoid.MaxHealth / 2
if game.Players.LocalPlayer.Character.Humanoid.Health <= yellowzone then
    script.Parent.BackgroundColor3 = Color3.new(255,255,0)
end
if game.Players.LocalPlayer.Character.Humanoid.Health > yellowzone then
local brickcolor = BrickColor.new("Lime green")
local color = brickcolor.Color
script.Parent.BackgroundColor3 = color
end
if game.Players.LocalPlayer.Character.Humanoid.Health <= redzone then
script.Parent.BackgroundColor3 = Color3.new(255,0,0)
end
if game.Players.LocalPlayer.Character.Humanoid.Health > redzone then
local brickcolor = BrickColor.new("Lime green")
local color = brickcolor.Color
script.Parent.BackgroundColor3 = color
end
script.Parent.Size = UDim2.new(game.Players.LocalPlayer.Character.Humanoid.Health / game.Players.LocalPlayer.Character.Humanoid.MaxHealth,0,0,20)
end)

-- Gradually regenerates the Humanoid's Health over time.

local REGEN_RATE = 1/100 -- Regenerate this fraction of MaxHealth per second.
local REGEN_STEP = 1 -- Wait this long between each regeneration step.

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local Character = script.Parent
local Humanoid = Character:WaitForChild'Humanoid'

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while true do
    while Humanoid.Health < Humanoid.MaxHealth do
        local dt = wait(REGEN_STEP)
        local dh = dt*REGEN_RATE*Humanoid.MaxHealth
        Humanoid.Health = math.min(Humanoid.Health + dh, Humanoid.MaxHealth)
    end
    Humanoid.HealthChanged:Wait()
end