Error in requests.post

[snippsat]

Have you inspect login (dev-tools Chrome or FireFox),to see what happens when login?
The Form Data most be correct,and you may need a some Header data.
json.dumps(payload) is not needed Requests do serializing automatic as this is build in.

With login there are many different methods and it easy to get stuck.
Requests doc Authentication has some about basic ones,eg Digest, OAuth 1 and 2,OpenId ect...

If i want to log in into this site.

import requests
from bs4 import BeautifulSoup

headers = {
    'User-Agent': 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/68.0.3440.106 Safari/537.36'
}

params = {
    "username": "user_name",
    "password": "xxxxxxxxx",
    "remember": "yes",
    "submit": "Login",
    "action": "do_login",
}

with requests.Session() as s:
    s.post('https://python-forum.io/member.php?action=login', headers=headers, params=params)
    # logged in! cookies get saved for future requests
    response = s.get('https://python-forum.io/index.php')
    # cookies sent automatically!
    soup = BeautifulSoup(response.content, 'lxml')
    welcome = soup.find('span', class_="welcome").text
    print(welcome)

Output:
Welcome back, snippsat. You last visited: Today, 07:29 PM Log Out