So i created this Damage display script, but it works only once and after that time it errors?

You only created one instance of an Attachment object. On line 29, you destroy that object the first time .Touched fires, and therefore there are no more attachments to destroy (hence the NULL error). To fix this, create a new attachment whenever the .Touched event fires.

local isUsing = false
local tool = script.Parent.Parent
local Blade1 = script.Parent.Parent.Union
local Blade2 = script.Parent.Parent.Union2
local Blades = Blade1 or Blade2
local DamageDis = script.Parent.Parent.Instances.DamageDisplay:Clone()
local Damage = script.Parent.Parent.Values.Damage
local DamageText = script.Parent.Parent.Instances.DamageDisplay.Damage:Clone()
local CanDamage = script.Parent.Parent.Values.CanDamage

tool.Activated:Connect(function()
isUsing = true
wait(2)
isUsing = false
end)

Blades.Touched:Connect(function(hit)
    local HumanoidRoot = hit.Parent:FindFirstChild("HumanoidRootPart")
      if isUsing and HumanoidRoot and CanDamage.Value == true then
        local Attachment = Instance.new("Attachment")
        Attachment.Parent = HumanoidRoot
        DamageDis.Parent = Attachment
        DamageText.Parent = DamageDis
        DamageText.Text = tostring(Damage.Value)
        DamageDis.Enabled = true
        wait(2)
        DamageDis:Destroy()
        DamageText:Destroy()
        Attachment:Destroy()
  end
end)