[Django] django.db.transaction.TransactionManagementError

# user.tests
class UserSignUpTest(TestCase):
    def setUp(self):
        self.client = Client()
        User.objects.create(
            nickname='user1',
            phone='010111110000',
            password=bcrypt.hashpw('user1111'.encode('utf-8'), bcrypt.gensalt()).decode('utf-8'),
            is_staff=False
        )
        User.objects.create(
            nickname='admin1',
            phone='01099999999',
            password=bcrypt.hashpw('admin1234'.encode('utf-8'), bcrypt.gensalt()).decode('utf-8'),
            is_staff=True
        )

    def tearDown(self):
        User.objects.all().delete()
    
    def test_fail_duplicate_phone(self):
        data = {
            "nickname": "user3",
            "phone": "010111110000",
            "password": "user3333",
            "is_staff": False
        }
        response = self.client.post("/user/signup/", data=data, content_type="application/json")
        self.assertEqual(response.status_code, 400)
        self.assertEqual(response.json(), {"message": "DUPLICATE_PHONE"})

# user.views
class SignUpView(APIView):
    @swagger_auto_schema(tags=["Sign Up"], request_body=UserSerializer, responses={201: 'Success'})
    def post(self, request):
        try:
            data = json.loads(request.body)

            if check_length(data["nickname"], 2):
                return JsonResponse({"message": "NICKNAME_SHORT"}, status=400)

            if check_length(data["phone"], 10):
                return JsonResponse({"message": "PHONE_SHORT"}, status=400)

            if check_password(data["password"]):
                return JsonResponse({"message": "INVALID_PASSWORD"}, status=400)

            User(
                phone=data["phone"],
                nickname=data["nickname"],
                password=hash_password(data["password"]).decode("utf-8"),
                is_staff=data["is_staff"]
            ).save()

            return HttpResponse(status=201)

        except IntegrityError:
            return JsonResponse({"message": "DUPLICATE_PHONE"}, status=400)
        except KeyError:
            return JsonResponse({"message": "INVALID_KEYS"}, status=400)