What's wrong with the serializer?

Question

Django

My models

class Address(models.Model):
    street = models.CharField(max_length=100)
    city = models.CharField(max_length=100)
    zip_code = models.CharField(max_length=15)

class User(AbstractUser):
    phone = models.CharField(max_length=16, blank=True, null=True)

    shipping_address = models.ForeignKey(Address, on_delete=models.CASCADE, blank=True, null=True)
...

My serializer

class FinishUserRegistrationSerializer(serializers.HyperlinkedModelSerializer):
    street = serializers.ReadOnlyField(source='address.street')
    city = serializers.ReadOnlyField(source='address.city')
    zip_code = serializers.ReadOnlyField(source='address.zip_code')

    class Meta:
        model = User
        fields = (
            'street',
            'city',
            'zip_code',
            'email',
            'paypal_address',
            'ebay_id',
            'resellers_permit_number',
        )

    def update(self, instance, validated_data):
        address = Address.objects.create(
            street=validated_data['street'],
            city=validated_data['city'],
            zip_code=validated_data['zip_code']
        )
        instance.shipping_address = address
        instance.paypal_address = validated_data('paypal_address', instance.paypal_address)
        instance.ebay_id = validated_data('ebay_id', instance.ebay_id)
        instance.resellers_permit_number = validated_data('resellers_permit_number', instance.resellers_permit_number)
        instance.save()
        return instance

And my view

class UserFinishRegistrationView(GenericAPIView):
    serializer_class = FinishUserRegistrationSerializer

    def post(self, request):
        serializer = FinishUserRegistrationSerializer(data=request.data)
        if serializer.is_valid(raise_exception=True):
            user = User.objects.get(username=request.data['email'])
            if user:
                address = Address.objects.create(street=request.data['street'],
                                                 city=request.data['city'],
                                                 zip_code=request.data['zip_code'])
                json = serializer.data
                user.paypal_address = request.data['paypal_address']
                user.ebay_id = request.data['ebay_id']
                user.resellers_permit_number = request.data['resellers_permit_number']
                user.save()
                return Response(json, status=status.HTTP_201_CREATED)
            return Response(serializers.get_error_detail, status=status.HTTP_400_BAD_REQUEST)
        return Response(serializers.get_error_detail, status=status.HTTP_400_BAD_REQUEST)

Everything works fine, the address is created. Only for some reason does not assign it to the user. It is this line that does not work instance.shipping_address = address
. Through the shell, all the rules. What's wrong?