Internal Server Error 500 Google OAuth Flask

I am trying to add login using google. I am using Oauth to achieve the same.

I am being re directed to the google login in page but upon logging in the during the call back I am constantly running into this error("Internal Server Error: The server encountered an internal error and was unable to complete your request. Either the server is overloaded or there is an error in the application.") & ("Invalid Client Error" in valid_token_paramerters unauthorized in the terminal) and after going through tons of articles I still unable to get through.

I am relatively new to this so I am unable to check as to where its going wrong.

Disclosure: I am using Flask for this app. GOOGLE_CLIENT_ID & GOOGLE_CLIENT_SECRET are assigned environment variables.

Here are the files: app.py

# Python standard libraries
import json
import os
import sqlite3

# Third-party libraries
from flask import Flask, redirect, request, url_for
from flask_login import (
    LoginManager,
    current_user,
    login_required,
    login_user,
    logout_user,
)
from oauthlib.oauth2 import WebApplicationClient
import requests

# Internal imports
from db import init_db_command
from user import User


# Configuration
GOOGLE_CLIENT_ID = os.environ.get("GOOGLE_CLIENT_ID", None)
GOOGLE_CLIENT_SECRET = os.environ.get("GOOGLE_CLIENT_SECRET", None)
GOOGLE_DISCOVERY_URL = (
    "https://accounts.google.com/.well-known/openid-configuration"
)

# Flask app setup
app = Flask(__name__)
app.secret_key = os.environ.get("SECRET_KEY") or os.urandom(24)

# User session management setup
# https://flask-login.readthedocs.io/en/latest
login_manager = LoginManager()
login_manager.init_app(app)

# Naive database setup
try:
    init_db_command()
except sqlite3.OperationalError:
    # Assuming it's already been created
    pass

# OAuth 2 client setup
client = WebApplicationClient(GOOGLE_CLIENT_ID)

# Flask-Login helper to retrieve a user from our db
@login_manager.user_loader
def load_user(user_id):
    return User.get(user_id)

@app.route("/")
def index():
    if current_user.is_authenticated:
        return (
            "<p>Hello, {}! You're logged in! Email: {}</p>"
            "<div><p>Google Profile Picture:</p>"
            '<img src="{}" alt="Google profile pic"></img></div>'
            '<a class="button" href="/logout">Logout</a>'.format(
                current_user.name, current_user.email, current_user.profile_pic
            )
        )
    else:
        return '<a class="button" href="/login">Google Login</a>'

def get_google_provider_cfg():
    return requests.get(GOOGLE_DISCOVERY_URL).json()


@app.route("/login")
def login():
    #What URL to hit for google login
    google_provider_cfg = get_google_provider_cfg()
    authorization_endpoint = google_provider_cfg["authorization_endpoint"]
    request_uri = client.prepare_request_uri(
        authorization_endpoint,
        redirect_uri=request.base_url + '/callback',
        scope=["openid","email","profile"]
    )
    return redirect(request_uri)


@app.route("/login/callback")
def callback():
    # Get the unique authorization code Google sent back to client
    code = request.args.get("code")
    # Finding the URL to hit to get tokens that will allow the app to ask for details on behalf of user
    google_provider_cfg = get_google_provider_cfg()
    token_endpoint = google_provider_cfg["token_endpoint"]

    # Preparing token request
    token_url, headers, body = client.prepare_token_request(
        token_endpoint,
        authorization_response=request.url,
        code=code
    )
    token_response = requests.post(
        token_url,
        headers=headers,
        data=body,
        auth=(GOOGLE_CLIENT_ID,GOOGLE_CLIENT_SECRET)
    )

    # Pasing the tokens
    client.parse_request_body_response(json.dumps(token_response.json()))

    # After getting tokens, hititng the URL from google that gives us the User info
    userinfo_endpoint = google_provider_cfg["userinfo_endpoint"]
    uri, headers, body = client.add_token(userinfo_endpoint)
    userinfo_response = requests.get(uri,headers=headers,data = body)

    # Getting user info if the user's email is verified
    if userinfo_response.json().get("email_verified"):
        unique_id = userinfo_response.json()["sub"]
        users_email = userinfo_response.json()["email"]
        picture = userinfo_response.json()["picture"]
        users_name = userinfo_response.json()["given_name"]
    else:
        return "User email is not available or verified by Google.", 400


    # Creating user in Db with info from Google
    user = User( 
        id_=unique_id,name=users_name,email=users_email,profile_pic=picture
    )

    # Adding user to database if user doesnt exist
    if not User.get(unique_id):
        User.create(unique_id,users_name,users_email,picture)

    # Begin user session by loggin in 
    login_user(user)

    # Sending it back to Home after authnetication 
    return redirect(url_for("index"))

@app.route("/logout")
@login_required
def logout():
    logout_user()
    return redirect(url_for("index"))


if __name__ == "__main__":
    app.run(ssl_context="adhoc")

requirements.txt

requests==2.21.0
Flask==1.0.2
oauthlib==3.0.1
pyOpenSSL==19.0.0
Flask-Login==0.4.1

launch.json

{
    "name": "Python: Flask",
    "type": "python",
    "request": "launch",
    "module": "flask",
    "env": {
        "FLASK_APP": "application.py",
        "FLASK_ENV": "development",
        "FLASK_DEBUG": "0"
    },
    "args": [
        "run",
        "--no-debugger",
        "--no-reload"
    ],
    "jinja": true
}

Thanks in advance.