Type Error: expected string or bytes-like object in python

I am facing an error that occurs whenever I try to python manage.py migrate. It says:

  Apply all migrations: admin, aufgabenzettel, auth, contenttypes, sessions
Running migrations:
  Applying aufgabenzettel.0005_auto_20210526_1527...Traceback (most recent call last):
  File "C:\Users\adam_\Documents\Coding\My_First\Aufgabenliste\manage.py", line 22, in <module>        
    main()
  File "C:\Users\adam_\Documents\Coding\My_First\Aufgabenliste\manage.py", line 18, in main
    execute_from_command_line(sys.argv)
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\core\management\__init__.py", line 401, in execute_from_command_line
    utility.execute()
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\core\management\__init__.py", line 395, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\core\management\base.py", line 330, in run_from_argv
    self.execute(*args, **cmd_options)
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\core\management\base.py", line 371, in execute
    output = self.handle(*args, **options)
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\core\management\base.py", line 85, in wrapped
    res = handle_func(*args, **kwargs)
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\core\management\commands\migrate.py", line 243, in handle
    post_migrate_state = executor.migrate(
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\db\migrations\executor.py", line 117, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)   
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\db\migrations\executor.py", line 147, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\db\migrations\executor.py", line 227, in apply_migration
    state = migration.apply(state, schema_editor)
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\db\migrations\migration.py", line 124, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\db\migrations\operations\fields.py", line 104, in database_forwards
    schema_editor.add_field(
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\db\backends\sqlite3\schema.py", line 328, in add_field
    self._remake_table(model, create_field=field)
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\db\backends\sqlite3\schema.py", line 189, in _remake_table
    self.effective_default(create_field)
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\db\backends\base\schema.py", line 303, in effective_default
    return field.get_db_prep_save(self._effective_default(field), self.connection)
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\db\models\fields\__init__.py", line 823, in get_db_prep_save
    return self.get_db_prep_value(value, connection=connection, prepared=False)
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\db\models\fields\__init__.py", line 1378, in get_db_prep_value
    value = self.get_prep_value(value)
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\db\models\fields\__init__.py", line 1357, in get_prep_value
    value = super().get_prep_value(value)
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\db\models\fields\__init__.py", line 1217, in get_prep_value
    return self.to_python(value)
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\db\models\fields\__init__.py", line 1318, in to_python
    parsed = parse_datetime(value)
  File "C:\Users\adam_\AppData\Local\Programs\Python\Python39\lib\site-packages\django\utils\dateparse.py", line 107, in parse_datetime
    match = datetime_re.match(value)
TypeError: expected string or bytes-like object

I am quite confused and don't know what to do about it...

Here is my code:

urls.py

from django.urls import path

from . import views

urlpatterns = [
    path("", views.index, name="index"),
    path("<int:aufgabenzettel_id>", views.details, name="details"),
    path("add/", views.add, name="add"),
    path("delete/<int:aufgabenzettel_id>", views.delete, name="delete"),
    path("edit/<int:aufgabenzettel_id>", views.edit, name="edit"),
    path("update/<int:aufgabenzettel_id>", views.update, name="update")
]

views.py

from django.http.response import HttpResponseRedirect
from django.shortcuts import render
from django.urls import reverse
from django.utils import timezone
from datetime import datetime

from .models import Aufgabenzettel

# Create your views here.
def index(request):
    Aufgabe_alert = Aufgabenzettel.Geplant
    now = timezone.now()
    if Aufgabe_alert == now:
        print("Timer abgelaufen!")
    Aufgaben_items = Aufgabenzettel.objects.all().order_by("-Geplant") #Aufgaben nach Datum ordnen (letzte oben durch minus)
    return render(request, "aufgabenzettel/index.html", {
        "Aufgabenliste":Aufgabenzettel.objects.all(), #für die foor loop in index wichtig GEWESEN; obsolet
        "Aufgaben_items":Aufgaben_items
    })

def details(request, aufgabenzettel_id):
    aufgabenzettel = Aufgabenzettel.objects.get(pk=aufgabenzettel_id)
    creationDate = aufgabenzettel.Datum
    dueDate = aufgabenzettel.Geplant
    return render(request, "aufgabenzettel/details.html", {
        "details":aufgabenzettel,
        "creationDate": creationDate,
        "dueDate":dueDate
    })

def add(request):
    addDatum = timezone.now()
    if request.method == "POST":
        Aufgabe = request.POST["Hinzufügen"]
        geplantes_datum = request.POST["DatumFeld"]
        Aufgabenzettel.objects.create(Aufgabeselbst=Aufgabe , Datum=addDatum, Geplant=geplantes_datum) #Aufgabenname und Aufgabendatum erstellen
        return HttpResponseRedirect(reverse("index"))
    return render(request, "aufgabenzettel/add.html")

def delete(request, aufgabenzettel_id):
    aufgabenzettel = Aufgabenzettel.objects.get(pk=aufgabenzettel_id)
    aufgabenzettel.delete()
    return HttpResponseRedirect(reverse("index"))

def edit(request, aufgabenzettel_id):
    aufgabenzettel = Aufgabenzettel.objects.get(pk=aufgabenzettel_id)
    return render(request, "aufgabenzettel/edit.html", {
        "details":aufgabenzettel
    })

def update(request, aufgabenzettel_id):
    if request.method == "POST":
        Aufgabe = Aufgabenzettel.objects.get(pk=aufgabenzettel_id)
        Aufgabe.Aufgabeselbst = request.POST["Bearbeiten"]
        Aufgabe.save()
        return HttpResponseRedirect(reverse("index"))
    return render(request, "aufgabenzettel/edit.html")

models.py

from django.db import models

# Create your models here.
class Aufgabenzettel(models.Model):
    Aufgabeselbst = models.CharField(max_length=64)
    Datum = models.DateTimeField(auto_now_add=True)
    Geplant = models.DateTimeField(auto_now_add=False, auto_now=False)

    def __str__(self):
        return f"{self.Aufgabeselbst}"

layout.html

{% load static %}
<!DOCTYPE html>
<html lang="de">
    <head>
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <title>Aufgabenzettel</title>
        <link rel="stylesheet" href="{% static '/css/main.css' %}">
    </head>
    <body>
        {% block body %}
        {% endblock %}
    </body>

</html>

index.html

{% extends "aufgabenzettel/layout.html" %}

{% block body %}
<h1 id="MeineAufgaben">Meine Aufgaben</h1>
    <table>
        {% for Aufgabeselbst in Aufgaben_items %}
            <tr>
                <td>
                    <a href="{% url 'details' Aufgabeselbst.id %}"> 
                        {{ Aufgabeselbst }}
                    </a>
                </td>
                <td>
                    <form action="{% url 'delete' Aufgabeselbst.id %}" method="post">
                        {% csrf_token %}
                    <button type="submit"  id="löschenbtn">Löschen</button>
                </form>
                </td>
                <td>
                    <form action="{% url 'edit' Aufgabeselbst.id %}" method="post">
                        {% csrf_token %}
                    <button type="submit" value="{{ details }}" class="bearbeitenbtn">Bearbeiten</button>
                    </form>
                </td>
            </tr>
        {% endfor %}
    </table>
    <h2>
        <a href="{% url 'add' %}" id="neuebtn">Neue Aufgabe erstellen</a>
    </h2>
{% endblock %}

add.html

{% extends "aufgabenzettel/layout.html" %}

{% block body %}
    <h1>Füge eine neue Aufgabe hinzu</h1>
    <form action="{% url 'add' %}" method="post">
        {% csrf_token %}
        <input type="text" name="Hinzufügen" placeholder="Neue Aufgabe">
        <input type="datetime-local" name="DatumFeld">
        <button type="submit" id="Hinzufügen">Hinzufügen</button>
    </form>
{% endblock %}

details.html

{% extends "aufgabenzettel/layout.html" %}

{% block body %}
    <h1>{{ details }}</h1>
    <h3>Erstellt: {{ creationDate }}</h3>
    <h2>Geplant: {{ dueDate }}</h2>
    <a href="{% url 'index' %}">Zurück zu Aufgabe</a>
{% endblock %}

edit.html

{% extends "aufgabenzettel/layout.html" %}

{% block body %}
    <h2>Bearbeite deine Aufgabe "{{ details }}"</h2>
    <form action="{% url 'update' details.id %}" method="post">
    {% csrf_token %}
        <input type="text" name="Bearbeiten" value="{{details}}">
        <button type="submit" class="bearbeitenbtn">Bearbeiten</button>
    </form>
    <a href="{% url 'index' %}">Zurück zu Aufgabe</a>
{% endblock %}

If I run python manage.py runserver it works fine in the browser but at the same time my terminal says You have 3 unapplied migration(s). Your project may not work properly until you apply the migrations for app(s): aufgabenzettel. Run 'python manage.py migrate' to apply them. . I then executed python manage.py makemigrations and all the changes were applied. However, if I try python manage.py migrate the TypeError occurs. I don't know what to do about it and appreciate every kind of help!