I am participating in a course and in it an activity was just to copy and paste the code provided and to use postman, but when I did this I got this error that is in the title, follow the code

Minha views.py

from django.views.decorators.csrf import csrf_exempt
from rest_framework.response import Response
from rest_framework import status
from .models import Livro
from .serializers import LivroSerializer

@csrf_exempt
def livro_list_create(request):
    if request.method == 'GET':
        livros = Livro.objects.all()
        serializer = LivroSerializer(livros, many=True)
        return Response(serializer.data)

    if request.method == 'POST':
        serializer = LivroSerializer(data=request.data)
        if serializer.is_valid():
            serializer.save()
            return Response(serializer.data, status=status.HTTP_201_CREATED)
        return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)

@csrf_exempt
def livro_detail(request, pk):
    livro = Livro.objects.get(pk=pk)

    if request.method == 'GET':
        serializer = LivroSerializer(livro)
        return Response(serializer.data)

    if request.method == 'PUT':
        serializer = LivroSerializer(livro, data=request.data)
        if serializer.is_valid():
            serializer.save()
            return Response(serializer.data)
        return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)

    if request.method == 'DELETE':
        livro.delete()
        return Response(status=status.HTTP_204_NO_CONTENT)

urls.py

from django.urls import path
from .views import livro_list_create, livro_detail

urlpatterns = [
    path('livros/', livro_list_create, name='livros-list-create'),
    path('livros/<int:pk>/', livro_detail, name='livro-detail'),
]

serializer.py

from rest_framework import serializers
from .models import Categoria, Autor, Livro

class CategoriaSerializer(serializers.ModelSerializer):
    class Meta:
        model = Categoria
        fields = ['id', 'nome']

class AutorSerializer(serializers.ModelSerializer):
    class Meta:
        model = Autor
        fields = ['id', 'nome']

class LivroSerializer(serializers.ModelSerializer):
    class Meta:
        model = Livro
        fields = ['id', 'titulo', 'autor', 'categoria', 'publicado_em']

    def update(self, instance, validated_data):
        instance.titulo = validated_data.get('titulo', instance.titulo)
        instance.autor = validated_data.get('autor', instance.autor)
        instance.categoria = validated_data.get('categoria', instance.categoria)
        instance.publicado_em = validated_data.get('publicado_em', instance.publicado_em)
        instance.save()
        return instance

class LivroSerializer(serializers.Serializer):
    id = serializers.IntegerField(read_only=True)
    titulo = serializers.CharField(max_length=200)
    autor = serializers.PrimaryKeyRelatedField(queryset=Autor.objects.all())
    categoria = serializers.PrimaryKeyRelatedField(queryset=Categoria.objects.all())
    publicado_em = serializers.DateField()

    def create(self, validated_data):
        return Livro.objects.create(**validated_data)

    def update(self, instance, validated_data):
        instance.titulo = validated_data.get('titulo', instance.titulo)
        instance.autor = validated_data.get('autor', instance.autor)
        instance.categoria = validated_data.get('categoria', instance.categoria)
        instance.publicado_em = validated_data.get('publicado_em', instance.publicado_em)
        instance.save()
        return instance

class CategoriaSerializer(serializers.Serializer):
    id = serializers.IntegerField(read_only=True)
    nome = serializers.CharField(max_length=100)

    def create(self, validated_data):
        return Categoria.objects.create(**validated_data)

    def update(self, instance, validated_data):
        instance.nome = validated_data.get('nome', instance.nome)
        instance.save()
        return instance

class AutorSerializer(serializers.Serializer):
    id = serializers.IntegerField(read_only=True)
    nome = serializers.CharField(max_length=100)

    def create(self, validated_data):
        return Autor.objects.create(**validated_data)

class CategoriaSerializer(serializers.ModelSerializer):
    class Meta:
        model = Categoria
        fields = ['id', 'nome']

class AutorSerializer(serializers.ModelSerializer):
    class Meta:
        model = Autor
        fields = ['id', 'nome']

class LivroSerializer(serializers.ModelSerializer):
    class Meta:
        model = Livro
        fields = ['id', 'titulo', 'autor', 'categoria', 'publicado_em']

    def update(self, instance, validated_data):
        instance.nome = validated_data.get('nome', instance.nome)
        instance.save()
        return instance

class LivroSerializer(serializers.Serializer):
    id = serializers.IntegerField(read_only=True)
    titulo = serializers.CharField(max_length=200)
    autor = serializers.PrimaryKeyRelatedField(queryset=Autor.objects.all())
    categoria = serializers.PrimaryKeyRelatedField(queryset=Categoria.objects.all())
    publicado_em = serializers.DateField()

    def create(self, validated_data):
        return Livro.objects.create(**validated_data)

    def update(self, instance, validated_data):
        instance.titulo = validated_data.get('titulo', instance.titulo)
        instance.autor = validated_data.get('autor', instance.autor)
        instance.categoria = validated_data.get('categoria', instance.categoria)
        instance.publicado_em = validated_data.get('publicado_em', instance.publicado_em)
        instance.save()
        return instance

I was only able to solve this problem by using @api_view() before the functions and placing the methods they receive, but this code appears in the activity, so I wanted to understand what is wrong with it for this error to occur.