Why does the function with vk api buttons not work?

Question

Python

Hello!

Faced such a problem: when I replace the button code with the main_menu () function, an error appears.

Error:
Traceback (most recent call last):
File "C:\Users\Alex\Desktop\mybot\problem.py", line 41, in
write(sender, "Hi")
File "C:\Users\Alex\Desktop \mybot\problem.py", line 10, in write
vk.method("messages.send",{'user_id': sender,'message': message,"random_id": get_random_id(), 'keyboard': keyboard. get_keyboard()})
File "C:\Users\aleksey\AppData\Local\Programs\Python\Python38-32\lib\site-packages\vk_api\vk_api.py", line 656, in method
raise error
vk_api.exceptions.ApiError: [911] Keyboard format is invalid: row 0 contains too many columns

This part of the code is:

keyboard = VkKeyboard(one_time=True)
    def write(sender, message):    #функция для ответа в диалоге 
        vk.method("messages.send",{'user_id': sender,'message': message,"random_id": get_random_id(), 'keyboard': keyboard.get_keyboard()})
    def main_menu():
        keyboard = VkKeyboard(one_time=True)
        keyboard.add_button('Баланс', color=VkKeyboardColor.SECONDARY)
        keyboard.add_button('Услуги', color=VkKeyboardColor.SECONDARY)
        keyboard.add_line()
        keyboard.add_button('Тарифы', color=VkKeyboardColor.SECONDARY)
        keyboard.add_button('Роуминг', color=VkKeyboardColor.SECONDARY)
        keyboard.add_line()
        keyboard.add_button('Вызвать оператора', color=VkKeyboardColor.PRIMARY)
   
    for event in longpoll.listen():   
        if event.type == VkEventType.MESSAGE_NEW and event.to_me and event.text:
            text = event.text
            sender = event.user_id
            if b==0:
                main_menu()
                #keyboard = VkKeyboard(one_time=True)
                #keyboard.add_button('Баланс', color=VkKeyboardColor.SECONDARY)
                #keyboard.add_button('Услуги', color=VkKeyboardColor.SECONDARY)
                #keyboard.add_line()
                #keyboard.add_button('Тарифы', color=VkKeyboardColor.SECONDARY)
                #keyboard.add_button('Роуминг', color=VkKeyboardColor.SECONDARY)
                #keyboard.add_line()
                #keyboard.add_button('Вызвать оператора', color=VkKeyboardColor.PRIMARY)
                write(sender, "Привет")

Before the replacement everything works fine. What is the reason for this and how can I fix it?