Attempted to call NIL Value error when I run this script?

wait(5)
Start()

function Start()
	print("This function has ran!")
if game.Players.NumPlayers >= 2 then 
	Yes()
else
	No()	
end
end

function Yes()
	print("This function has ran!")
	wait(.1)
end

function No()
	print("This function has ran!")
	music = game.Workspace.HeartofCourage
	wait(1)
	music:stop()
	wait(5)
	music:play()
	music.Looped = true 
end

I have tried to fix it. This script is fricken' simple and it isn't working? Oh my god, imagine a complex script. Please tell me how I can fix this.

What it exactly says in the output window:

19:45:42.795 - Stack Begin
19:45:42.795 - Script 'Workspace.X', Line 2
19:45:42.796 - Stack End

Function definitions are actually local variable assignments.

The following two snippets are equivalent:

local a = function()
	...
end

function a()
	...
end

Looking at your code, then, we see:

-- Line 2, source of error:
[Some use of Start]

-- Line 4, definition of Start

Which is clearly a problem, because you are using something before it was ever defined (using its default value of nil; specifically, calling its default value of nil because this is a function call).

The call to Start() has to occur after Start and and functions that Start references (Yes and No) have been defined, just like any code referencing variables has to be after the definition of those variables.