The URL is incorrect and you are missing the form data, you also need to complete the initial request, send it to the correct URL, and then get http://pro.wialon.com/service.html:
data = {"user": "demo",
"passw": "demo",
"submit": "Enter",
"lang": "en",
"action": "login"}
head = {"User-Agent":"Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36"}
with requests.Session() as c:
c.get('http://pro.wialon.com/')
url = 'http://pro.wialon.com/login_action.html'
c.post(url, data=data, headers=head)
print(c.get("http://pro.wialon.com/service.html").content)
You can see the message in chrome dev tools on the network tab:

post get , .
:
<form class="login_bg_form" id="login_form" action="login_action.html" method="POST">
, bs4:
import requests
from bs4 import BeautifulSoup
from urlparse import urljoin
data = {"user": "demo",
"passw": "demo",
"submit": "Enter",
"lang": "en",
"action": "login"}
head = {"User-Agent":"Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36"}
with requests.Session()as c:
soup = BeautifulSoup(c.get('http://pro.wialon.com/').content)
redir = soup.select_one("#login_form")["action"]
url = 'http://pro.wialon.com/login_action.html'
c.post(url, data=data, headers=head)
print(c.get(urljoin("http://pro.wialon.com/", redir)).content)
, ajax, , , .