Ошибка jinja2.exceptions.UndefinedError flask

у меня есть проблема с проектом на flask.У меня имеется каркас сайта , на главной странице которого выводится список.

При переходе нажатии на один из элементов этого списка(а их тут будет много) происходит переход на его страницу, где должен отображаться ещё один список. Однако выводится такая ошибка.

Уже несколько дней бьюсь над этой проблемой и не могу понять - в чем причина.

Код на python:

import sqlite3

from flask import Flask, render_template, request, redirect, flash, url_for
from flask_login import LoginManager, logout_user
from flask_sqlalchemy import SQLAlchemy
from werkzeug.security import check_password_hash, generate_password_hash

app = Flask(__name__)
app.secret_key = '/=q3|40~$<l?6($!g$0|'
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///site.db'
app.config['SQLALCHEMY_DATABASE_MODIFICATIONS'] = False
db = SQLAlchemy(app)
manager = LoginManager(app)


class User(db.Model):
    id = db.Column(db.Integer, autoincrement=True)
    fname = db.Column(db.String(50), primary_key=True, nullable=False)
    sname = db.Column(db.String(50), primary_key=True, nullable=False)
    age = db.Column(db.Integer, nullable=False)
    email = db.Column(db.String(100), primary_key=True, nullable=False)
    password = db.Column(db.String(100), nullable=False)

    def __repr__(self):
        return '<User %r>' % self.id


class Article(db.Model):
    id = db.Column(db.Integer, autoincrement=True)
    name_music = db.Column(db.String(100), primary_key=True, nullable=False)
    cfname = db.Column(db.String(50), nullable=False)
    csname = db.Column(db.String(50), nullable=False)
    notes = db.Column(db.String(100), nullable=False)
    cmark = db.Column(db.Float, nullable=False)

    class Columnes(db.Model):
        id = db.Column(db.Integer, autoincrement=True)
        fname = db.Column(db.String(50), nullable=False)
        sname = db.Column(db.String(50), nullable=False)
        mark = db.Column(db.Float, nullable=False)
        age = db.Column(db.Integer, nullable=False)
        link = db.Column(db.String(100), primary_key=True, nullable=False)
        link_image = db.Column(db.String(100), nullable=False)
        name_music = db.Column(db.String(100), nullable=False)

        def __repr__(self):
            return '<Columnes %r>' % self.id

    a = list()
    sorted(a, key=lambda column: column.mark)

    def __repr__(self):
        return '<Article %r>' % self.id


@manager.user_loader
def load_user(user_id):
    return User.query.get(user_id)


@app.route('/')
@app.route('/sign-up', methods=['POST', 'GET'])
def sign_up():
    if request.method == "POST":
        fname = request.form["fname"]
        sname = request.form["sname"]
        age = request.form["age"]
        email = request.form["email"]
        password = request.form["password"]
        rep_password = request.form["rep_password"]
        if not (fname or sname or age or email or password or rep_password):
            flash("Please , fill all fields!")
        elif len(password) < 6 or password != rep_password:
            flash("Password is incorrect!")
        else:
            hash1 = generate_password_hash(password)
            user = User(fname=fname, sname=sname, age=age, email=email, password=hash1)
            db.session.add(user)
            db.session.commit()
            return redirect('/home')
    else:
        return render_template('sign-up.html')


@app.route('/home')
def home():  # put application's code here
    articles = Article.query.order_by(Article.cmark.desc()).all()
    return render_template('index.html', articles=articles)


@app.route('/composition/<string:name>')
def composition(name):
    name_music = name
    article = Article.query.order_by(Article.cmark.desc()).all()
    per = 0
    for el in article:
        if el.name_music == name_music:
            per = el
    return render_template('composition.html', comp=per)


@app.route('/create-col', methods=['POST', 'GET'])
def create_col():
    if request.method == 'POST':
        fname = request.form["fname"]
        sname = request.form["sname"]
        age = request.form["age"]
        mark = request.form["mark"]
        name_music = request.form["name_music"]
        link = request.form["link"]
        link_image = "https://i.ytimg.com/vi/" + link[32:] + "/mqdefault.jpg"
        article = Article.query.order_by(Article.cmark.desc()).all()
        column = Article.Columnes(sname=sname, fname=fname, link=link, link_image=link_image, age=age, mark=mark,
                                  name_music=name_music)
        for el in article:
            if el.name_music == name_music:
                el.a.append(column)
                break
        db.session.commit()
        return redirect('/home')
    else:
        return render_template('column.html')


@app.route('/login_page')
def login_page():
    return render_template('base.html')


@app.route('/logout')
def logout():
    logout_user()
    return redirect('/home')


@app.route('/login', methods=['POST', 'GET'])
def check_login():
    if request.method == "POST":
        email = request.form["email"]
        password = request.form["password"]
        sqlconnection = sqlite3.Connection("site.db")
        cursor = sqlconnection.cursor()
        query1 = "SELECT email , password from User WHERE email='{email}' AND password='{password}'".format(
            email=email, password=password)
        rows = cursor.execute(query1)
        rows = rows.fetchall()
        if len(rows) == 1:
            user = User.query.filter_by(email=email).first
            if user and check_password_hash(user.password, password):
                load_user(user)

                next1 = request.args.get('next')
                redirect(next1)
            else:
                flash('Login or password is not correct')
        else:
            return redirect('/login')
    else:
        flash('Please fill login and password fields!')
    return render_template('login.html')


@app.route('/create-article', methods=['POST', 'GET'])
def create_article():
    if request.method == "POST":
        cmark = request.form["c_mark"]
        mark = request.form["mark"]
        cfname = request.form["c_fname"]
        csname = request.form["c_sname"]
        fname = request.form["fname"]
        sname = request.form["sname"]
        name_music = request.form["name_music"]
        link = request.form["link"]
        link_image = "https://i.ytimg.com/vi/" + link[32:] + "/mqdefault.jpg"
        age = request.form["age"]
        notes = request.form["notes"]
        article = Article(cfname=cfname, csname=csname, cmark=cmark, notes=notes, name_music=name_music)
        column = Article.Columnes(sname=sname, fname=fname, link=link, link_image=link_image, age=age, mark=mark,
                                  name_music=name_music)
        article.a.append(column)
        db.session.add(article)
        db.session.add(column)
        db.session.commit()
        return redirect('/home')
    else:
        return render_template('create-article.html')


@app.after_request
def redirect_to_signin(response):
    if response.status_code == 401:
        return redirect(url_for('check_login') + '?next=' + request.url)
    return response


if __name__ == '__main__':
    app.run(debug=True)

И если понадобиться - мой код html, выводящий страничку определенного элемента списка:

{% extends 'base.html' %}
{% block body %}
    <h1 style="text-align: center">{{ comp.name_music }}</h1>
    <h2 style="text-align: center">Author: {{ comp.cfname }} {{ comp.csname}} , Verified by {{ comp.a[0].fname }} {{ comp.a[0].sname }}</h2>
    <h2 style="text-align: center">Difficulty : {{ comp.cmark }}</h2>
    <h2 style="margin-left: 300px">The link to sheets: <a href="{{ comp.notes }}" style="text-decoration: none;">Sheets</a></h2>

    <h1 style="margin-left: 100px">The top:</h1>
    {% for el in comp.a %}
    <div class="otstup">
         <div class="list-group list-group-vertical justify-content-center">
             <li class="list-group-item d-flex justify-content-between align-items-start element-list">
                 <div class="container-sm2">
                     <a href="{{ el.link }}">
                         <img src="{{ el.link_image }}" alt="erhg" width="140px" height="90px">
                     </a>
                 </div>
                 <div class="container-sm">
                     <br>
                    <h3>Исполнитель: {{ el.fname }}  {{ el.sname }}</h3>
                    <h4>Возраст: {{ el.age }}</h4>
                 </div>
             </li>
         </div>
    </div>
    {% endfor %}
{% endblock %}

Upd:вторая часть ошибки: