Why does this script in ScreenGUI not want to work in online mode?

The code you're using for your function is very, very unnecessary; you don't need to go that far, as it's really, really simple: all you need to do is use the for loop, like the one you're using, but w/o all those calculations & if statements.

All you require, as you used, is the for loop & the sub function:

local SomeStringToUse = 'Stringy-string!!' -- Our string, which will be used for the following for loop; variable "Welcome" represents the string

for x = 1, SomeStringToUse:len() do -- Starting position, and end position; "SomeStringToUse:len()" returns the number of characters there're in "SomeStringToUse," or the length of the string: you can also use "#SomeStringToUse" to accomplish this as well
	print(SomeStringToUse:sub(1, x)) -- Will print the string as the substring increases.
	wait(1 / 44) -- Yields the code for 1-44th of a second
end -- Ends code

And when you fire this, it'll look like this in the Output:

--[[
	S
	St
	Str
	Stri
	Strin	
	String
	Stringy
--]]

And so on; it'll work (more than likely) as you intended your code to be like.

You over-complicated something simple, but that's all right, we're only human. :)

Stuff touched on but didn't go into great detail about

1. Output

2. Sub Strings

3. For loop

4. If statement

Hope this helped you in any way. :)

Edit Just read the title... Oops. ;-;

Your problem is that Tick may not be existent at the time of execution of the code; a way to counter this is to use the handy-dandy WaitForChild function!

• Choir * (OoO)

It'll look similar to this:

Parent:WaitForChild('Child_Name')

What WaitForChild does is yield the code, and listens for when the child is existent w/in the specified parent; the parent is require, b/c that's where it's going to wait for the child in, and so is the argument (the name of the child), b/c it'll use that to wait & find the child/ object.

I hope this helped you in any way. :)