@api_view(["POST"])
def register_view(request: "Request"):
    serializer = RegisterSerializer(data=request.data)

    serializer.is_valid(raise_exception=True)

    try:
        serializer.save()
    except IntegrityError:
        raise ConflictException

    return Response(serializer.data, status=status.HTTP_201_CREATED)

class ConflictException(APIException):
    status_code = 409
    default_detail = "username is already taken"
    default_code = "conflict"

    def test_username_taken(self):
        with transaction.atomic():
            User.objects.create_user(username="customer4", password="Passw0rd!")

        response = self.client.post(
            "/register",
            {
                "username": "customer4",
                "password": "Passw0rd!",
                "confirm_password": "Passw0rd!",
                "phone_number": "13324252666"
            },
        )
        self.assertEqual(response.status_code, status.HTTP_409_CONFLICT)

The test result shows :

raise TransactionManagementError(
django.db.transaction.TransactionManagementError: An error occurred in the current transaction. You can’t execute queries until the end of the ‘atomic’ block.

I’m a Django and DRF noob and don’t know what to do…

I tested on postman and there is no problem, with 409 status_code response