Defining function difficulties ["NameError: name 'number' is not defined"] - python-3.x

Question

Okay, trying to make a simple game of Guessing Numbers but I can't find the mistake in this code. Still pretty new to python so probably the reason why but I can't figure out what is wrong with it.
import random
from time import sleep
def start():
print("Welcome To The Guessing Game \n Try to guess the number I'm thinking of \n Good luck!")
selectRandomNumber()
guessCheck(number, numberInput=1)
def restart():
print("Creating new number ...")
sleep(1)
print("OK")
selectRandomNumber()
guessCheck(number,numberInput=1)
def selectRandomNumber():
number = random.randint(0,1000)
tries = 0
return
def tryAgain():
while True:
try:
again = int(input("Do you want to play again? y/n:"))
except ValueError:
print("Couldn't understand what you tried to say")
continue
if again == "y" or "yes":
print("Awesome! Lets go")
restart()
elif again == 'n' or "no":
print("Goodbye!")
break
else:
print("Not a valid option")
continue
def guessCheck(number,numberInput=1):
while True:
try:
numberInput = int(input("What number do you think it is?: "))
except ValueError:
print("Couldn't understand that. Try again")
continue
if numberInput > number:
print("Too high")
tries += 1
continue
elif numberInput < number:
print("Too low")
tries += 1
continue
elif numberInput == number:
print("Congrats! You got my number")
tryAgain()
number = selectRandomNumber()
print(number)
start()
Every time I try to run the program I keep getting the same mistake.
It tells me:
Traceback (most recent call last):
File "python", line 60, in <module>
start()
File "python", line 8, in start
guessCheck(number, numberInput)
NameError: name 'number' is not defined
Don't quite understand what that means.
Some help would be appreciated. Thanks!
* UPDATE *
Was able to fix the part about defining the variable but now new problem happened where when I try to run
Same code as before but added
guessCheck(number,numberInput=1)
and also added the variable number at the end
number = selectRandomNumber()
print(number)
start()
when I run it I get this
None # this is from `print(number)` so instead of getting a number here I'm getting `None`
Welcome To The Guessing Game
Try to guess the number I'm thinking of
Good luck!
What number do you think it is?:

Answer 1

The Traceback is telling you this:
We got to start().
start() called guessCheck().
We tried to pass two pieces of information to guessCheck(): the variable names number and numberInput.
We don't have those variables defined yet! numberInput doesn't get defined until once we've already started guessCheck(), and number isn't actually defined anywhere.
As Manoj pointed out in the comments, you probably want number to hold the output of selectRandomNumber(). So, instead of just calling selectRandomNumber() in start(), try number = selectRandomNumber() instead.
You can add a print(number) on the line right after that to make sure number has a value assigned to it.
Now number has a value, going into your call to guessCheck(). That still leaves numberInput undefined though. You can set a default value for function arguments like this:
guessCheck(number, numberInput=1)
That way, when guessCheck is called but numberInput hasn't been defined yet, it will automatically give it the value 1 until you set it explicitly.
You may encounter other issues with your code the way it is. My advice would be to start really simply - build up your game from each individual piece, and only put the pieces together when you're sure you have each one working. That may seem slower, but trying to go too fast will cause misunderstandings like this one.