roblox simple transparency animation?

Your code isn't looping because you haven't told it to. Depending on how many times you wish to loop, there are different functions supported by Lua to achieve this.

For Loops

Let's say you wanted to loop the code 10 times. For this, you would have to wrap it within a for loop to instruct the script to repeat this code over 10 increments.

Here is an example of a for loop in its most basic form. You can see that i is set to 1 as the start integer, and the end integer is set to 10.

for i=1, 10 do
	print("This code was repeated " .. i .. " times!")
end

For what you're trying to achieve, all you'd need to do is add the loops already written inside another loop!

See the example code below

local Part = script.Parent
local go = true -- I didn't see this used anywhere, but I've kept it for consistency

for i=1, 10 do -- This loop controls how many times the code is run in total.

	for i = 0,1.05,0.05 do -- This loop makes the part invisible
		Part.Transparency = i
		wait(0.1)
		if Part.Transparency == 1 then
			wait(5)
		end
	end
	
	for i = 1.2,-0.05,-0.05 do -- This loop makes the part visible again
		Part.Transparency = i
		wait(0.1)
		if Part.Transparency == 0 then
			wait(4)
		end
	end
	
	print("Script has looped " .. i .. " times!")
end

But let's say you wanted to run this code an unlimited number of times (or until a condition is met), for that we can use...

While Loops

Much like how we've just demonstrated how to use for loops with your code, all you'd need to do is add the code inside a while loop.

A while loop is a loop that will run until a condition is met. This could be a variable, or for a never-ending loop, something such as 1 + 1 = 2 would suffice as it always results as true. On that note, simply using true as your condition would also work.

Here's a few examples using what we just talked about...

while 1 + 1 = 2 do
	print("The loop ran!")
end

while true do
	print("The loop ran!")
end

For your code, you could use either of these, or any of the many loop varieties available on the Developer Hub (https://developer.roblox.com/en-us/articles/Loops).

Hope this helps!