How come this script welds the model to the right leg, but not the left leg?

The script only welds the copied model to the right leg, but not the left, here is the script:

-- Begin Function
function onTouched(hit)
	-- RightLeg
	local RightLeg = hit.Parent["Right Leg"]
	if hit.Parent:findFirstChild("Humanoid") ~= nil and RightLeg:findFirstChild("RocketBoot") == nil then
		local g = game.Lighting.RocketBoot:Clone()
		g.Parent = RightLeg
		local C = g:GetChildren()
		for i=1, #C do
			if C[i].className == "Part" or C[i].ClassName == "UnionOperation" then
				local W = Instance.new("Weld")
				W.Part0 = g.Middle
				W.Part1 = C[i]
				local CJ = CFrame.new(g.Middle.Position)
				local C0 = g.Middle.CFrame:inverse()*CJ
				local C1 = C[i].CFrame:inverse()*CJ
				W.C0 = C0
				W.C1 = C1
				W.Parent = g.Middle
			end
				local Y = Instance.new("Weld")
				Y.Part0 = RightLeg
				Y.Part1 = g.Middle
				Y.C0 = CFrame.new(0, 0, 0.05) * CFrame.fromEulerAnglesXYZ(0, 0, 0)
				Y.Parent = Y.Part0
		end

		local h = g:GetChildren()
		for i = 1, # h do
			if h[i]:IsA("Part") then
				h[i].Anchored = false
				h[i].CanCollide = false
				h[i].TopSurface = "SmoothNoOutlines"
				h[i].BottomSurface = "SmoothNoOutlines"
				h[i].LeftSurface = "SmoothNoOutlines"
				h[i].RightSurface = "SmoothNoOutlines"
				h[i].FrontSurface = "SmoothNoOutlines"
				h[i].BackSurface = "SmoothNoOutlines"
			end
		end
	end	

	--LeftLeg
	local LeftLeg = hit.Parent["Left Leg"]
	if hit.Parent:findFirstChild("Humanoid") ~= nil and LeftLeg:findFirstChild("RocketBoot") == nil then
		
		local g = game.Lighting.RocketBoot:Clone()
		g.Parent = LeftLeg
		local C = g:GetChildren()
		for i=1, #C do
			if C[i].className == "Part" or C[i].ClassName == "UnionOperation" then
				local W = Instance.new("Weld")
				W.Part0 = g.Middle
				W.Part1 = C[i]
				local CJ = CFrame.new(g.Middle.Position)
				local C0 = g.Middle.CFrame:inverse()*CJ
				local C1 = C[i].CFrame:inverse()*CJ
				W.C0 = C0
				W.C1 = C1
				W.Parent = g.Middle
			end
				local Y = Instance.new("Weld")
				Y.Part0 = RightLeg
				Y.Part1 = g.Middle
				Y.C0 = CFrame.new(0, 0, 0.05) * CFrame.fromEulerAnglesXYZ(0, 0, 0)
				Y.Parent = Y.Part0
		end

		local h = g:GetChildren()
		for i = 1, # h do
			if h[i]:IsA("Part") then
				h[i].Anchored = false
				h[i].CanCollide = false
				h[i].TopSurface = "SmoothNoOutlines"
				h[i].BottomSurface = "SmoothNoOutlines"
				h[i].LeftSurface = "SmoothNoOutlines"
				h[i].RightSurface = "SmoothNoOutlines"
				h[i].FrontSurface = "SmoothNoOutlines"
				h[i].BackSurface = "SmoothNoOutlines"
			end
		end
	end	
end
-- Ending first function
script.Parent.Touched:connect(onTouched)

They're at this place: http://www.roblox.com/Steampunk-place?id=296134153

And this is the output: `20:10:50.007 - Right Leg is not a valid member of Model

20:10:50.008 - Script 'Workspace.TheButton.Script', Line 5

20:10:50.009 - Stack End`

I believe the error you've showed us is misleading (not blaming you) -- that error suggests that hit.Parent has no Right Leg (which is easy enough to do -- any non-character model that comes into contact with it will cause the script to error).

I'm not sure as to the actual cause of your problem, but if you use functions to clean up your duplicate code, you might find it easier to debug (ex. by putting print statements through it). Further, if you find a bug, you then only have to change it in the one place, rather than two.

Also, you can remove the error you did find by using :FindFirstChild to get the Right/Left legs (and simply not doing that leg if it doesn't exist).

function WeldBootToLeg(leg)
	--lines 6 - 39 here
end
function onTouched(hit)
    -- RightLeg
    local RightLeg = hit.Parent:FindFirstChild("Right Leg")
	if RightLeg and hit.Parent:FindFirstChild("Humanoid") ~= nil and RightLeg:FindFirstChild("RocketBoot") == nil then
		WeldBootToLeg(RightLeg)
	end

    --LeftLeg
    local LeftLeg = hit.Parent:FindFirstChild("Left Leg")
    if LeftLeg and hit.Parent:FindFirstChild("Humanoid") ~= nil and LeftLeg:FindFirstChild("RocketBoot") == nil then
		WeldBootToLeg(LeftLeg)
	end
end

script.Parent.Touched:connect(onTouched)

Either another error will pop up (allowing you to address that one), or nothing will show up. If the latter, then start adding print statements (probably in the WeldBootToLeg function) to see which lines are running. ex, you might put print(leg, #) (replacing '#' with a unique number (1,2,...) so that you know which print statements are running). Try to not put any print statements inside for loops that will iterate a lot of times, or else you'll flood your Output window.