Magento 2 : Create shipment from order:

order-to-shipment.php

/**
 * @var OrderRepositoryInterface
 */
protected $orderRepository;

/**
 * @var \Magento\Sales\Model\Convert\Order
 */
protected $convertOrder;

/**
 * @var ShipmentRepositoryInterface
 */
protected $shipmentRepository;

/**
 * Order constructor.
 * @param OrderRepositoryInterface $orderRepository
 * @param ShipmentRepositoryInterface $shipmentRepository
 * @param \Magento\Sales\Model\Convert\Order $convertOrder
 */
public function __construct(
    OrderRepositoryInterface $orderRepository,
    ShipmentRepositoryInterface $shipmentRepository,
    \Magento\Sales\Model\Convert\Order $convertOrder
)
{
    $this->orderRepository = $orderRepository;
    $this->shipmentRepository = $shipmentRepository;
    $this->convertOrder = $convertOrder;
}

/**
 * @param int $orderId
 * @return \Magento\Sales\Model\Order\Shipment
 * @throws LocalizedException
 */
public function createShipmentFromOrderId(int $orderId)
{
    /** @var \Magento\Sales\Model\Order $order */
    $order = $this->orderRepository->get($orderId);

    if (!$order->canShip()) {
        throw new LocalizedException(__('Order cannot be shipped.'));
    }

    // Create the shipment:
    $shipment = $this->convertOrder->toShipment($order);

    // Add items to shipment:
    foreach ($order->getAllItems() as $orderItem) {
        if (!$orderItem->getQtyToShip() || $orderItem->getIsVirtual()) {
            continue;
        }

        $qtyShipped = $orderItem->getQtyToShip();
        $shipmentItem = $this->convertOrder->itemToShipmentItem($orderItem)->setQty($qtyShipped);
        $shipment->addItem($shipmentItem);
    }

    // Register the shipment:
    $shipment->register();

    try {
        $this->shipmentRepository->save($shipment);
        $this->orderRepository->save($shipment->getOrder());
    } catch (\Exception $e) {
        throw new LocalizedException(__($e->getMessage()));
    }

    return $shipment;
}