Problem Serial port connexion and save file voltage variable

[HAMMOUTI]

from machine import ADC
import time
import os
import io
adc = ADC()

bat_voltage = adc.channel(attn=ADC.ATTN_11DB, pin='P16')

vbat = bat_voltage.voltage()
 
with open("vbatmv.txt", "a+") as f:
 while True :
    bat_voltage = adc.channel(attn=ADC.ATTN_11DB, pin='P16')
    vbat = bat_voltage.voltage()
    # note that the expansionboard 3 has a voltage divider of 1M / 1M to account for
    # 1M / 1M, ratio = 1:2
    vbat = vbat*2
    
    time.sleep(30)
    
    print('battery voltage:', vbat, 'mV')
    
    print(vbat, file=f)

Hi, I can you help me to solve this problem , I try to save voltage value in file with print command some time work other not , and i have problem to connect tool upycraft to my pycom board 3.1 serial port in execution of this programme .