serial output not display in tkinter label

Question

from cgitb import text

from distutils.cmd import Command

import tkinter as tk

from socket import timeout

import time

import pyfirmata

import time

from time import sleep, strftime

from tkinter import Tk, Label

from tkinter import Tk, Label

import random

import os 

import tkinter.font as tkFont

ports="COM4"

board = pyfirmata.Arduino('COM4')  

analog_pin0 = board.get_pin('a:1:i') 

x=0

it = pyfirmata.util.Iterator(board)  

it.start()

pin=4

analog_pin0.enable_reporting() 

board.digital[7].write(0)

voltage0 = "" 



def main():
   print("this is maoin")



def on():
      global voltage0
      root = tk.Tk()
      root.geometry("600x500")
 

      w = tk.Label(root, text="Hello, world!")
      w.pack(pady=20)  
      # B = tk.Button(root, text ="Hello", command = main) 
      # B.pack()
      w.config(text=voltage0)
      w['text'] = "intial voltage"

      B = tk.Button(root, text ="Hello", command = main)
      B.pack() 
      root.mainloop()  

      def main ():
         while True:
     
            global voltage0
            reading0 = analog_pin0.read()
            if reading0 != None:
               voltage0 = reading0 * 22.3
               voltage0 = round(voltage0,2)
               time.sleep(0.1)
               print(voltage0)
               w.config(text=voltage0)
               w['text'] = voltage0
               
               
            else:
               voltage0 = float('nan')
            
            
            
            
            string = strftime('%H:%M:%S %p')
                  
            
            
            if (voltage0>7.3):
               time.sleep(0.1)  
               board.digital[pin].write(1)
               board.digital[6].write(0) 
               
            if(voltage0<6.5):
                  time.sleep(0.1)  
                  board.digital[pin].write(0)
                  board.digital[6].write(1) 

            

            if (voltage0==7.3):
               
               
               print()
               print(voltage0)
               print(string)
               w.config(text=voltage0)
               w['text'] = voltage0

            if (voltage0==6.5):
               
               print()
               print(voltage0)
               print(string)  
               w.config(text=voltage0)
               w['text'] = voltage0
      
      main()
     
on()