Gunakan CreateDBSubnetGroup dengan AWS SDK

    /**
     * Creates a new DB subnet group asynchronously.
     *
     * @param groupName the name of the subnet group to create
     * @return a CompletableFuture that, when completed, returns the Amazon Resource Name (ARN) of the created subnet group
     * @throws CompletionException if the operation fails, with a cause that may be a ServiceQuotaExceededException if the request would exceed the maximum quota
     */
    public CompletableFuture<String> createSubnetGroupAsync(String groupName) {

        // Get the Amazon Virtual Private Cloud (VPC) where the Neptune cluster and resources will be created
        String vpcId = getDefaultVpcId();
        logger.info("VPC is : " + vpcId);

        List<String> subnetList = getSubnetIds(vpcId);
        for (String subnetId : subnetList) {
            System.out.println("Subnet group:" +subnetId);
        }

        CreateDbSubnetGroupRequest request = CreateDbSubnetGroupRequest.builder()
                .dbSubnetGroupName(groupName)
                .dbSubnetGroupDescription("Subnet group for Neptune cluster")
                .subnetIds(subnetList)
                .build();

        return getAsyncClient().createDBSubnetGroup(request)
                .whenComplete((response, exception) -> {
                    if (exception != null) {
                        Throwable cause = exception.getCause();
                        if (cause instanceof ServiceQuotaExceededException) {
                            throw new CompletionException("The operation was denied because the request would exceed the maximum quota.", cause);
                        }
                        throw new CompletionException("Failed to create subnet group: " + exception.getMessage(), exception);
                    }
                })
                .thenApply(response -> {
                    String name = response.dbSubnetGroup().dbSubnetGroupName();
                    String arn = response.dbSubnetGroup().dbSubnetGroupArn();
                    logger.info("Subnet group created: " + name);
                    return arn;
                });
    }

def create_subnet_group(neptune_client, group_name: str):
    """
    Creates a Neptune DB subnet group and returns its name and ARN.

    Args:
        neptune_client (boto3.client): The Neptune client object.
        group_name (str): The desired name of the subnet group.

    Returns:
        tuple(str, str): (subnet_group_name, subnet_group_arn)

    Raises:
        RuntimeError: For quota errors or other AWS-related failures.
    """
    vpc_id = get_default_vpc_id()
    subnet_ids = get_subnet_ids(vpc_id)

    request = {
        'DBSubnetGroupName': group_name,
        'DBSubnetGroupDescription': 'My Neptune subnet group',
        'SubnetIds': subnet_ids,
        'Tags': [{'Key': 'Environment', 'Value': 'Dev'}]
    }

    try:
        response = neptune_client.create_db_subnet_group(**request)
        sg = response.get("DBSubnetGroup", {})
        name = sg.get("DBSubnetGroupName")
        arn = sg.get("DBSubnetGroupArn")

        if not name or not arn:
            raise RuntimeError("Response missing subnet group name or ARN.")

        print(f"Subnet group created: {name}")
        print(f"ARN: {arn}")
        return name, arn

    except ClientError as e:
        code = e.response["Error"]["Code"]
        msg = e.response["Error"]["Message"]

        if code == "ServiceQuotaExceededException":
            print("Subnet group quota exceeded.")
            raise RuntimeError("Subnet group quota exceeded.") from e
        else:
            print(f"AWS error [{code}]: {msg}")
            raise RuntimeError(f"AWS error [{code}]: {msg}") from e

    except Exception as e:
        print(f"Unexpected error creating subnet group '{group_name}': {e}")
        raise RuntimeError(f"Unexpected error creating subnet group '{group_name}': {e}") from e