So the whole thing basically works – i can Register a new user, i can login i can get the shares quotes etc.
But as soon as i try to buy a share, i get an error: ValueError: NOT NULL constraint failed: portfolio.shares.
CODE:

@app.route("/buy", methods=["GET", "POST"])
@login_required
def buy():
    """Buy shares of stock"""

    # if request method is GET, display buy.html form
    if request.method == "GET":
        return render_template("buy.html")

    # if request method is POST
    else:
        # save stock symbol, number of shares, and quote dict from form
        symbol = request.form.get("symbol")
        shares = request.form.get("shares")
        quote = lookup(symbol)

        # return apology if symbol not provided or invalid
        if quote == None:
            return apology("must provide valid stock symbol", 400)

        # return apology if shares not provided or invalid
        if not shares or not shares.isdigit() or int(shares) <= 0:
            return apology("must provide a positive number of shares", 400)

        # cast symbol to uppercase and cast shares to int, in order to work with them
        symbol = symbol.upper()
        shares = int(shares)
        purchase = quote['price'] * shares

        # make sure user can afford current stock, checking amount of cash in users table

        # select this user's cash balance from users table
        balance = db.execute("SELECT cash FROM users WHERE id = :id", id=session["user_id"])
        balance = balance[0]['cash']
        remainder = balance - purchase

        # if purchase price exceeds balance, return error
        if remainder < 0:
            return apology("insufficient funds", 400)

        # query portfolio table for row with this userid and stock symbol:
        row = db.execute("SELECT * FROM portfolio WHERE userid = :id AND symbol = :symbol",
                         id=session["user_id"], symbol=symbol)

        # if row doesn't exist yet, create it but don't update shares
        if len(row) != 1:
            db.execute("INSERT INTO portfolio (userid, symbol) VALUES (:id, :symbol)",
                       id=session["user_id"], symbol=symbol)

        # get previous number of shares owned
        oldshares = db.execute("SELECT shares FROM portfolio WHERE userid = :id AND symbol = :symbol",
                               id=session["user_id"], symbol=symbol)
        oldshares = oldshares[0]["shares"]

        # add purchased shares to previous share number
        newshares = oldshares + shares

        # update shares in portfolio table
        db.execute("UPDATE portfolio SET shares = :newshares WHERE userid = :id AND symbol = :symbol",
                   newshares=newshares, id=session["user_id"], symbol=symbol)

        # update cash balance in users table
        db.execute("UPDATE users SET cash = :remainder WHERE id = :id",
                   remainder=remainder, id=session["user_id"])

        # update history table
        db.execute("INSERT INTO history (userid, symbol, shares, method, price) VALUES (:userid, :symbol, :shares, 'Buy', :price)",
                   userid=session["user_id"], symbol=symbol, shares=shares, price=quote['price'])

    # redirect to index page
    return redirect("/")

HTML:

{% extends "layout.html" %}

{% block title %}
    Buy
{% endblock %}

{% block main %}
    <form action="/buy" method="post">
        <div style="width:30%; margin: auto; class="form-group">
            <input class="form-control" name="symbol" placeholder="Symbol" type="text">
        </div>
        <div class="form-group">
            <input style="width:30%; margin: auto; class="form-control" name="shares" placeholder="Shares" type="number" min="0">
        </div>
        <button class="btn btn-primary" type="submit">Buy</button>
    </form>

{% endblock %}

.db

CREATE TABLE portfolio (
    id INTEGER PRIMARY KEY AUTOINCREMENT,
    userid INTEGER NOT NULL,
    symbol TEXT NOT NULL,
    shares INTEGER NOT NULL,
    FOREIGN KEY (userid) REFERENCES users(id)
);
CREATE TABLE history (
    id INTEGER PRIMARY KEY AUTOINCREMENT,
    userid INTEGER NOT NULL,
    symbol TEXT NOT NULL,
    shares INTEGER NOT NULL,
    price REAL NOT NULL,
    timestamp DATETIME DEFAULT CURRENT_TIMESTAMP,
    FOREIGN KEY (userid) REFERENCES users(id)
);

I googled, i asked my LLM, i searched in different forums, stackoverflow, reddit, ytb etc.

Along the way i was able to fix some earlier Problems but sadly on the check50 i still get this:

**:( buy handles valid purchase
    application raised an exception (see the log for more details)**
log:
sending GET request to /signin
sending POST request to /login
sending POST request to /buy
exception raised in application: ValueError: NOT NULL constraint failed: portfolio.shares

Please tell me if you need aditional information, i am thankfull for every input/help i get, i im stuck with this since over a week now 🙂

BG