In the output box it says Trail is missing from the ServerStorage yet it is there.Please help?

Scripts that load immediately often have errors like this because objects typically aren't instantly loaded in. The best way to fix this would be to use :WaitForChild to make sure that the script can see the object before you try to reference it. This should fix your problem:

local Trail = game.ServerStorage:WaitForChild("Trail")

game.Players.PlayerAdded:Connect(function(Player)
     Player.CharacterAdded:Connect(function(Character)
          local PlayerTrail = Trail:Clone()
          PlayerTrail.Parent = Character.Torso
          PlayerTrail.Attachment0 = Character.Head.FaceFrontAttachment
          PlayerTrail.Attachment1 = Character.Torso.WaistBackAttachment
     end)
end)

That's not how you get objects.

You should either do

local Trail = game.ServerStorage.Trail

or

local Trail = game.ServerStorage:WaitForChild("Trail")

But you don't use () parantheses to index an object. At most [] ones to evaluate the string you put inside them.