How do i fix refined in enclosing scope referenced before assignment of my code?

Question

I do not know why i can't add a number to a variable that is equal to a randomly generated number. i have tried a few dumb things like trying to assign a variable to Xplay. Didn't work. all I want to do is just randomly generate a x and y coordinates that will be the position of the rect and after that when ever I press a up, down, left and right key it will move by 10 pixels

import pygame, sys
from pygame.locals import *

import random

pygame.init()
DisplayWidth = 700
DisplayHeight = 400
BoxSize = 10

Black = (0, 0, 0)
DarkGreen = (0, 200, 0 )
Green = (0, 255, 0)
Red = (255, 0, 0)
White = (255, 255, 255)

start = True
DEATH = False

Xplay = random.randrange(2, 68)  #error's origin
Yplay = random.randrange(2, 38)  #error's origin
Xchange = 0
Ychange = 0




def main():
  global FPSClock, Display, Basicfont

  pygame.init()
  FPSClock = pygame.time.Clock()
  Display = pygame.display.set_mode((DisplayWidth, DisplayHeight))
  Basicfont = pygame.font.Font('freesansbold.ttf', 30)

  while start:
    RunGame()

    

def RunGame():
  pygame.draw.rect(Display, DarkGreen, (Xplay, Yplay, 20, 20))
  while not DEATH:
    for event in pygame.event.get():
      if event.type == QUIT:
        terminate()
      if event.type == pygame.KEYDOWN:
        if event.key == pygame.K_LEFT or pygame.K_a:
          Xchange = -10
          Ychange = 0
        elif event.key == pygame.K_UP or pygame.K_w:
          Xchange = 0
          Ychange = -10
        elif event.key == pygame.K_DOWN or pygame.K_s:
          Xchange = 0
          Ychange = 10
        elif event.key == pygame.K_RIGHT or pygame.K_d:
          Xchange = 10
          Ychange = 0  
    Xplay += Xchange   #error
    Yplay += Ychange   #error
    

      






def terminate():
  pygame.quit()
  sys.exit()