Python pymongo keeps refusing [Errno111] Connection refused, Timeout: 30s

Question

I tried to connect to MongoDB used pymongo. the log that comes out when print(db) is written was informed that the connection became normal.

// print(db) result
Database(MongoClient(host=['localhost:27017'], document_class=dict, tz_aware=False, connect=True), 'dbsparta')

But when i used to CURD method (ex : insertOne) -> [Errno 111] error occured...

// My Code
from pymongo import MongoClient 
client = MongoClient('localhost', 27017)  
db = client.localdb 

print(db)

db.users.insert_one({'name':'bobby','age':21})
db.users.insert_one({'name':'kay','age':27})
db.users.insert_one({'name':'john','age':30})

Err Message

raceback (most recent call last):
  File "/home/ek3434/Project/TIL/SWJungle/jungle.py", line 11, in <module>
    db.users.insert_one({'name':'bobby','age':21})
  File "/home/ek3434/Project/TIL/.venv/lib/python3.8/site-packages/pymongo/collection.py", line 705, in insert_one
    self._insert(document,
  File "/home/ek3434/Project/TIL/.venv/lib/python3.8/site-packages/pymongo/collection.py", line 620, in _insert
    return self._insert_one(
  File "/home/ek3434/Project/TIL/.venv/lib/python3.8/site-packages/pymongo/collection.py", line 609, in _insert_one
    self.__database.client._retryable_write(
  File "/home/ek3434/Project/TIL/.venv/lib/python3.8/site-packages/pymongo/mongo_client.py", line 1551, in _retryable_write
    with self._tmp_session(session) as s:
  File "/usr/lib/python3.8/contextlib.py", line 113, in __enter__
    return next(self.gen)
  File "/home/ek3434/Project/TIL/.venv/lib/python3.8/site-packages/pymongo/mongo_client.py", line 1948, in _tmp_session
    s = self._ensure_session(session)
  File "/home/ek3434/Project/TIL/.venv/lib/python3.8/site-packages/pymongo/mongo_client.py", line 1935, in _ensure_session
    return self.__start_session(True, causal_consistency=False)
  File "/home/ek3434/Project/TIL/.venv/lib/python3.8/site-packages/pymongo/mongo_client.py", line 1883, in __start_session
    server_session = self._get_server_session()
  File "/home/ek3434/Project/TIL/.venv/lib/python3.8/site-packages/pymongo/mongo_client.py", line 1921, in _get_server_session
    return self._topology.get_server_session()
  File "/home/ek3434/Project/TIL/.venv/lib/python3.8/site-packages/pymongo/topology.py", line 520, in get_server_session
    session_timeout = self._check_session_support()
  File "/home/ek3434/Project/TIL/.venv/lib/python3.8/site-packages/pymongo/topology.py", line 499, in _check_session_support
    self._select_servers_loop(
  File "/home/ek3434/Project/TIL/.venv/lib/python3.8/site-packages/pymongo/topology.py", line 218, in _select_servers_loop
    raise ServerSelectionTimeoutError(
pymongo.errors.ServerSelectionTimeoutError: localhost:27017: [Errno 111] Connection refused, Timeout: 30s, Topology Description: <TopologyDescription id: 614ae9c2b0ed5823d0a5f151, topology_type: Single, servers: [<ServerDescription ('localhost', 27017) server_type: Unknown, rtt: None, error=AutoReconnect('localhost:27017: [Errno 111] Connection refused')>]>

I've tried the way that comes out on this link(Pymongo keeps refusing the connection at 27017)

Implementing the solution given in that link results in the following results. but dosen't solve anything

// solution
sudo rm /var/lib/mongodb/mongod.lock
// err message
rm: cannot remove '/var/lib/mongodb/mongod.lock': No such file or directory

and

// solution
sudo service mongodb start
// error
 * Starting database mongod                     [fail] 

How can i fix it? I can't do anything because this error hasn't been resolved... T_T plz help..