updated_at = fields.DateTimeField(allow_null=True)

def validate_updated_at(self, value):

updated_at = fields.DateTimeField(allow_null=True)

def validate_updated_at(self, value):

class UserAnswerDeserializer(serializers.Serializer):
    uuid = fields.IntegerField()
    text = fields.CharField()
    state = fields.CharField()


class QuestionsAnswersDeserializer(serializers.Serializer):
    created_at = fields.DateTimeField()
    updated_at = fields.DateTimeField()
    uuid = fields.UUIDField()
    user_id = fields.IntegerField()
    scope = fields.CharField()
    state = UserAnswerDeserializer(many=True)
    question_text = fields.CharField()

    def validate_updated_at(self, value):
        print(value)
        return value if value else dt.datetime.now()


class AttemptsDeserializer(serializers.Serializer):
    user_id = fields.IntegerField()
    started_at = fields.DateTimeField()
    completed_at = fields.DateTimeField(default=now)
    status = fields.CharField()
    trajectory_id = fields.IntegerField()
    score = fields.FloatField()
    questions_answers = QuestionsAnswersDeserializer(many=True)

at = serializers.AttemptsDeserializer(data=attempt)

Данные, которые туда поступают

# attempt
{'user_id': 100500, 'started_at': '2000-01-01T01:01:01', 'completed_at': '2000-01-01T02:01:01', 'status': 'COMPLETED', 'trajectory_id': 2, 'score': 100, 'questions_answers': [{'created_at': '2000-01-01T01:01:01', 'updated_at': None, 'uuid': '1c387ed9-0fd7-4de1-b0a4-7219780274cd', 'user_id': 100500, 'scope': 'assessment', 'state': {'uuid': 2, 'text': 'answer 2'}, 'question_text': 'question-1'}]}

{'questions_answers': [{'updated_at': [ErrorDetail(string='This field may not be null.', code='null')], 'state': {'non_field_errors': [ErrorDetail(string='Expected a list of items but got type "dict".', code='not_a_list')]}}]}