What is wrong with my script & how can I learn from my mistakes in the future?

I'm working on a sequel to my 2D platformer game (Was a success obviously, I got 1 group fund from a 100 Robux Gamepass because that's math I guess) and I wanted to add a timer in this one. So I made the GUI, scripted it. But it's not working. And yes, I have checked the output. Nothing related to the script at all. 

Here is my script:

while true do
	wait(1)
	script.Parent.Valuf.Value = script.Parent.Valuf.Value - 1
	script.Parent.Text = script.Parent.Valuf.Value
end

What's even more important than fixing it is if you would like to, please explain how I can do this in the future! Subtracting int values in the studio for me is just a hit or miss. It's a 50/50 chance if it will work even if it is the same script.

"Give a man a fish, feed him for a day. Teach a man how to fish, feed him for life."