error

#! /usr/bin/env python
# -*- coding: utf-8 -*-

import vk_api, requests, random, json, datetime, re, time

'''session_user = requests.Session()
login, password = 'pass', 'pass'
vk_session = vk_api.VkApi(login, password)'''

vk_session = vk_api.VkApi(token='token')

from vk_api.longpoll import VkLongPoll, VkEventType
longpoll = VkLongPoll(vk_session)
vk = vk_session.get_api()
try:
    for event in longpoll.listen():
        if event.type == VkEventType.MESSAGE_NEW:
            if event.from_user:
                t=event.text
                t=t.encode('utf-8')
                id=str(event.user_id)
                now = datetime.datetime.utcnow()
                v=now.strftime("%d-%m-%Y %H:%M ")
                f = open("/root/out_from_user.txt", "a")
                f.write("@$#% " + v + " user_id: " + id + " soo: " + t + "\r\n")
                f.close()
except Exception as e:
    print str(e)


CODE OF ERROR:
Traceback (most recent call last):
  File "/root/Downloads/j.py", line 13, in <module>
    longpoll = VkLongPoll(vk_session)
  File "/usr/local/lib/python2.7/dist-packages/vk_api/longpoll.py", line 503, in __init__
    self.update_longpoll_server()
  File "/usr/local/lib/python2.7/dist-packages/vk_api/longpoll.py", line 513, in update_longpoll_server
    response = self.vk.method('messages.getLongPollServer', values)
  File "/usr/local/lib/python2.7/dist-packages/vk_api/vk_api.py", line 636, in method
    raise error
vk_api.exceptions.ApiError: [15] Access denied: no access to call this method