What is wrong with this decal-changing script?

Hi, So i'm writing this script that changes the decal texture asset ID of a part called "Screen". I've made loads of different stringvalues (one, two, three) which each have their own asset ID link (eg. http://www.roblox.com/asset/?id=238110849) i'm quite sure the links are ok. The hierarchy is:

Model, one, two, three, four, five, six, seven, eight, Part (ClickDetector, Script), Sign (PointLight, Decal)

Here's the script:

isOn = 0
local a=script.Parent.Parent.one.Value
local b=script.Parent.Parent.two.Value
local c=script.Parent.Parent.three.Value
local d=script.Parent.Parent.four.Value
local e=script.Parent.Parent.five.Value
local f=script.Parent.Parent.six.Value
local g=script.Parent.Parent.seven.Value
local h=script.Parent.Parent.eight.Value
local sign=script.Parent.Parent.Sign.Decal.Texture

function three()
do sign=""..a
	isOn=2
end
end

function four()
sign=""..b
	isOn=3
end

function five()
sign=""..c
	isOn=4
end

function six()
sign=""..d
	isOn=5
end
	
function seven()
sign=""..e
	isOn=6
end
	
function eight()
sign=""..f
	isOn=7
end

function two()
sign=""..g
	isOn=1
end

function nine()
sign=""..h
	isOn=0
end

function onClicked()
	
	
	if isOn == 0 then two() elseif isOn==1 then three() elseif isOn==2 then four() elseif isOn==3 then five() elseif isOn==4 then six() elseif isOn==5 then seven() elseif isOn==6 then eight() else nine() end

end

script.Parent.ClickDetector.MouseClick:connect(onClicked)

nine()

I know it's unnecessarily long but I made the original a while ago and just edited that.

Your problem is that you're attempting to edit a property directly from a variable

This won't work, you have to make the variable the object that the property is tied to, then edit the variable by indexing the property.

Also, your script has so many unnecessary aspects to it. Just make a table containing all of the values, then use your int variable to index the table.

local num =  0
local sign = script.Parent.Parent.Sign.Decal
local vals = { --Table full of all the values
script.Parent.Parent.one,
script.Parent.Parent.two,
script.Parent.Parent.three,
script.Parent.Parent.four,
script.Parent.Parent.five,
script.Parent.Parent.six,
script.Parent.Parent.seven,
script.Parent.Parent.eight
}

function incrementDecal()
	if num == #vals + 1 then num = 0 end --reset if it's at limit
	num = num + 1 --increment
	sign.Texture = vals[num].Value --Set the texture
end

script.Parent.ClickDetector.MouseClick:connect(function()
	incrementDecal()
end)

Your script is just over-complicated, which makes it easier to get errors. It would be much simpler to store all the string values into a table, and then having a variable that gets incremented by 1 every time the button is clicked. That way, you can use that variable to get the corresponding element in the table. Like so:

local Index = 1 --This is the index variable. It will be used to get the corresponding element from the table below
local Textures = { --This table contains the textureId's for the decals. You can also replace the "script.Parent.Parent.one.Value" with the actual value of One, and do the same for the rest of them
	script.Parent.Parent.one.Value;
	script.Parent.Parent.two.Value;
	script.Parent.Parent.three.Value;
	script.Parent.Parent.four.Value;
	script.Parent.Parent.five.Value;
	script.Parent.Parent.six.Value;
	script.Parent.Parent.seven.Value;
	script.Parent.Parent.eight.Value;
}
local Sign=script.Parent.Parent.Sign.Decal --The sign variable has to be the decal, not the texture of the decal because the texture of the decal is what you're trying to change. If Sign was the decal's texture, then whenever you said "Sign = ...", that would change the variable, not the texture

function onClicked()
	Sign.Texture = Textures[Index] --This sets the textureId of the decal to the Index'th element of the table "Textures"
	if Index == #Textures then --If the index value is the length of the table, then reset the Index value back to 1
		Index = 1
	else --If it's not, then just increment the Index value by 1
		Index = Index + 1
	end
end

script.Parent.ClickDetector.MouseClick:connect(onClicked)

Hope this helped!