Local scripts only works once caues i don't know what is the reason , HELP ?

Your issue is that you only check if the value of db is false once, and once you check if it is false, you set it to true. The only time you would set it back to false, according to the code you provided, would be if you hit a player. I would recommend changing your script to this:

local tool = script.Parent
local player = game:GetService("Players").LocalPlayer
local oGUI = script:FindFirstChild("ScreenGui")
local db = false


tool.Equipped:Connect(function(mouse)
    local GUI = oGUI:Clone()
    GUI.Parent = player.PlayerGui
    wait(1)
    GUI.Equiping.Visible = false
    mouse.Button1Down:connect(function()
        if not db == true then
            db = true
            local ray = Ray.new(tool.Glowstone.CFrame.p, (mouse.Hit.p - tool.Glowstone.CFrame.p).unit * 400)
            local part, position = workspace:FindPartOnRay(ray, player.Character, false, true)
            local beam = Instance.new("Part",workspace)
            beam.FormFactor = "Custom"
            beam.Material = "Ice"
            beam.BrickColor = BrickColor.new("Cyan")
            beam.CanCollide = false
            beam.Anchored = true
            beam.Transparency = 0.15
            beam.Locked = true
            local distance = (tool.Glowstone.CFrame.p - position).magnitude
            beam.Size = Vector3.new(0.3 , 0.3 , distance)
            beam.CFrame = CFrame.new(tool.Glowstone.CFrame.p, position) * CFrame.new(0,0, -distance /2)
            game:GetService("Debris"):AddItem(beam, 0.1)
            if part then
                local human = part.Parent:FindFirstChild("Humanoid")
                if not human then
                    human = part.Parent.Parent:FindFirstChild("Humanoid")
                end
                if human then
                   human:TakeDamage(3)
                   human.WalkSpeed = 1
                   wait(1)
                   GUI.Time.Visible = true
                   GUI.Time.Text = "4"
                   wait(1)
                   GUI.Time.Text = "3"
                   wait(1)
                   GUI.Time.Text = "2"
                   wait(1)
                   GUI.Time.Text = "1"
                   wait(1)
                   GUI.Time.Visible = false
                    wait(5)
                    db = false
                end
            end
        else -- You were missing this
            mouse.Button1Up:Connect(function()
                db = false
            end)
        end
    end)
end)