Check my app.py code. For some reason /sell is not properly rendering. I can't see the selector to sell my stocks. Here's my app.py code:
import os


from cs50 import SQL
from flask import Flask, flash, redirect, render_template, request, session
from flask_session import Session
from werkzeug.security import check_password_hash, generate_password_hash

from helpers import apology, login_required, lookup, usd

# Configure application
app = Flask(__name__)

# Custom filter
app.jinja_env.filters["usd"] = usd

# Configure session to use filesystem (instead of signed cookies)
app.config["SESSION_PERMANENT"] = False
app.config["SESSION_TYPE"] = "filesystem"
Session(app)

# Configure CS50 Library to use SQLite database
db = SQL("sqlite:///finance.db")


@app.after_request
def after_request(response):
    """Ensure responses aren't cached"""
    response.headers["Cache-Control"] = "no-cache, no-store, must-revalidate"
    response.headers["Expires"] = 0
    response.headers["Pragma"] = "no-cache"
    return response


@app.route("/")
@login_required
def index():
    # Assume db.execute() returns a dict or a list of dicts
        user_cash = db.execute("SELECT cash FROM users WHERE id = :id", id=session['user_id'])[0]['cash']
        purchases = db.execute("""
            SELECT symbol, SUM(shares) as shares, AVG(price) as price
            FROM purchases WHERE user_id = :id
            GROUP BY symbol
            HAVING SUM(shares) > 0;
            """, id=session['user_id'])
    # Fixed indent of for loop and calculate total
        for purchase in purchases:
            purchase['total'] = purchase['shares'] * purchase['price']


    # Moved the summation of total_assets out of the for loop
        total_assets = sum(purchase['total'] for purchase in purchases) + user_cash

        return render_template("index.html", cash=user_cash, purchases=purchases, total_assets=total_assets)




@app.route("/buy", methods=["GET", "POST"])
@login_required
def buy():
    if request.method == "GET":
        return render_template("buy.html")
    elif request.method == "POST":
        symbol = request.form.get('symbolb').upper()  # Get symbol and ensure it's uppercase
        try:
            quantity = int(request.form.get('quantity'))
        except ValueError:
            return apology("Invalid quantity", 400)

        if quantity <= 0:
            return apology("Quantity must be more than 0", 400)
        stock = lookup(symbol)
        if stock is None:
            return apology("Invalid stock symbol", 400)
        current_price = stock['price']

        user_cash = db.execute("SELECT cash FROM users WHERE id = :id", id=session['user_id'])[0]['cash']
        total_cost = current_price * quantity

        if total_cost > user_cash:
            return apology("Not enough cash to complete purchase", 403)

        db.execute("UPDATE users SET cash = cash - :cost WHERE id = :id",
            cost=total_cost, id=session['user_id'])
         # Insert the new purchase
        db.execute("INSERT INTO purchases (user_id, symbol, shares, price) VALUES (:user_id, :symbol, :shares, :price)",
                   user_id=session['user_id'], symbol=symbol, shares=quantity, price=current_price)

        flash("Purchase successful!")
        return redirect("/")


@app.route("/history")
@login_required
def history():
    """Show history of transactions"""
    return apology("TODO")


@app.route("/login", methods=["GET", "POST"])
def login():
    """Log user in"""

    # Forget any user_id
    session.clear()

    # User reached route via POST (as by submitting a form via POST)
    if request.method == "POST":

        # Ensure username was submitted
        if not request.form.get("username"):
            return apology("must provide username", 403)

        # Ensure password was submitted
        elif not request.form.get("password"):
            return apology("must provide password", 403)

        # Query database for username
        rows = db.execute("SELECT * FROM users WHERE username = ?", request.form.get("username"))

        # Ensure username exists and password is correct
        if len(rows) != 1 or not check_password_hash(rows[0]["hash"], request.form.get("password")):
            return apology("invalid username and/or password", 403)

        # Remember which user has logged in
        session["user_id"] = rows[0]["id"]

        # Redirect user to home page
        return redirect("/")

    # User reached route via GET (as by clicking a link or via redirect)
    else:
        return render_template("login.html")


@app.route("/logout")
def logout():
    """Log user out"""

    # Forget any user_id
    session.clear()

    # Redirect user to login form
    return redirect("/")


@app.route("/quote", methods=["GET", "POST"])
@login_required
def quote():
    """Get stock quote."""
    if request.method == "GET":
        return render_template("quote.html")
    elif request.method == "POST":
        my_symbol = request.form.get('symbol')
        if not my_symbol:
            flash("symbol is required", "error")
            return redirect("/quote")
        stock = lookup(my_symbol)
        if stock:
            return render_template("quoted.html", stock=stock)
        else:
            flash("Could not retrive stock info", "error")
            return redirect("/quote")
    else:
        flash("Invalid reuqest method", "error")
        return redirect("/quote")









@app.route("/quoted", methods=["GET", "POST"])
@login_required
def quoted():
    if request.method == "POST":
        return render_template("quoted.html", stock="stock")
    if request.method == "GET":
        return render_template("quoted.html", stock="stock")




@app.route("/register", methods=["GET", "POST"])
def register():

    # Forget any user_id
    session.clear()

    # User reached route via POST (as by submitting a form via POST)
    if request.method == "POST":

        # Ensure username was submitted
        if not request.form.get("username"):
            return apology("must provide username", 403)

        # Ensure password was submitted
        elif not request.form.get("password"):
            return apology("must provide password", 403)

        # Query database for username
        username = request.form.get("username")
        rows = db.execute("SELECT * FROM users WHERE username = :username", username=username)  # :username is a named placeholder
        if rows:
            return apology("username already exists", 403)

          # Hash user's password
        hashed_password = generate_password_hash(request.form.get("password"))

                # Insert new user into the database
        db.execute("INSERT INTO users (username, hash) VALUES (:username, :hash)", username=username, hash=hashed_password)  # :username and :hash are named placeholders


        # Redirect user to login page or some other page
        flash("Registered successfully, please log in.")
        return redirect("/login") # Assuming there is a login view

    else:  # User reached route via GET
        return render_template("register.html") # Assuming there is a 'register.html' template






@app.route("/sell", methods=["GET", "POST"])
@login_required
def sell():
    if request.method == "POST":
        symbol = request.form.get('symbol').upper()
        quantity = int(request.form.get('quantity'))

        if quantity <= 0:
            return apology("Can't sell less than 1 stonk!", 400)

        # Check how many shares the user currently has
        user_shares = db.execute("SELECT SUM(shares) as total_shares FROM purchases WHERE user_id = :user_id AND symbol = :symbol GROUP BY symbol",
                                 user_id=session['user_id'], symbol=symbol)

        if len(user_shares) != 1 or user_shares[0]['total_shares'] < quantity:
            return apology("Not enough shares to sell", 400)

        # Get the current price of the stock
        stock = lookup(symbol)

        total_sale = stock['price'] * quantity

        # Update user's shares and cash
        db.execute("UPDATE users SET cash = cash + :sale WHERE id = :id",
                   sale=total_sale, id=session['user_id'])
        db.execute("INSERT INTO purchases (user_id, symbol, shares, price) VALUES (:user_id, :symbol, :shares, :price)",
                   user_id=session['user_id'], symbol=symbol, shares=-quantity, price=stock['price'])

        flash("Sold successfully!")
        return redirect("/")

    else:  # GET request
        user_stocks = db.execute("SELECT symbol FROM purchases WHERE user_id = :user_id GROUP BY symbol HAVING SUM(shares) > 0",
                                 user_id=session['user_id'])
        return render_template('sell.html', user_stocks=user_stocks)

Here's my sell.html:
{% extends "layout.html" %}

{% block title %}
    Sell
{% endblock %}

{% block body %}
    <div class="mb-3">
        <form action="/sell" method="post">
            {% if user_stocks %}
                <div class="form-group">
                    <label for="symbol">Symbol:</label>
                    <select id="symbol" name="symbol" class="form-control">
                        {% for stock in user_stocks %}
                            <option value="{{ stock.symbol }}">{{ stock.symbol }}</option>
                        {% endfor %}
                    </select>
                </div>
            {% else %}
                <div class="alert alert-warning" role="alert">
                    You do not own any stocks to sell.
                </div>
            {% endif %}

            <div class="form-group">
                <label for="quantity">Quantity:</label>
                <input type="number" id="quantity" name="quantity" min="1" step="1" class="form-control">
            </div>

            <button type="submit" class="btn btn-primary">Sell</button>
        </form>
    </div>
{% endblock %}